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Firefox controls JPEG XL (.jxl) decoding with the Boolean preference image.jxl.enabled. Open about:config, search for that exact, case-sensitive name, and set it to true to enable decoding or false to disable it. The switch only works when your Firefox build contains the JPEG XL decoder, so a missing preference or an unchanged result can indicate a version or package limitation rather than a bad toggle.
What the Firefox JPEG XL setting does
JPEG XL is an image format that uses the .jxl file extension. Firefox’s decoder is governed by one Boolean preference: image.jxl.enabled. A value of true permits JPEG XL decoding in a build that includes the decoder; false turns decoding off. Mozilla identifies this preference in its developer documentation and Bugzilla records.
This is an image-decoding switch, not a converter. Enabling it does not turn existing JPEG or PNG files into JPEG XL files, add an operating-system codec, or make other applications open .jxl images. It only controls whether that Firefox installation attempts to decode JPEG XL content. If you need an image in a different format, you need a separate conversion tool; toggling this preference does not create a converted copy.
The setting is in Firefox’s Configuration Editor, commonly called about:config. Mozilla warns that changing advanced preferences can affect browser stability, security, and performance, and that these customizations are not officially supported. Keep the change limited to this one preference and reset it if Firefox behaves unexpectedly. The preference is useful for a focused compatibility test, but it is not a general-purpose performance or security control.
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Enable JPEG XL in Firefox
- Open the Configuration Editor. Enter
about:configin Firefox’s address bar and press Enter. - Accept the warning if shown. Firefox may display a “Proceed with Caution” or equivalent risk prompt before opening advanced preferences.
- Search for the exact preference. In the search field, enter
image.jxl.enabled. Preference names are case-sensitive, so use lower-case letters and the periods exactly as shown. - Inspect the current value. The row should identify the preference as a Boolean and show either
trueorfalse. - Set it to
true. Select the toggle button at the right of the row, or double-click the row, until the value readstrue. Mozilla documents both methods for Boolean preferences in its Configuration Editor instructions. - Retry the image. Reload the page containing the JPEG XL image, or close and reopen the image in Firefox. If the image was already cached or the page did not retry the request, use a hard reload or open the image URL in a new tab.
You normally do not need to restart Firefox for a preference toggle to take effect, but reopening the image or tab ensures that Firefox attempts decoding again. Check the value after toggling rather than relying on memory: if it still reads false, the decoder remains disabled by this preference.
Enabling the preference is only one part of the test. The build must include the decoder, and Firefox must receive a valid JPEG XL image. If the row changes to true but the image remains unavailable, avoid repeatedly switching the same value; move on to checking the build, the file, and the way the page delivers it.
Disable JPEG XL in Firefox
- Open
about:configand accept the warning if Firefox presents it. - Search for
image.jxl.enabled, using the exact spelling and capitalization. - Toggle the Boolean value until it reads
false. - Reload any page that displays a JPEG XL image, or reopen the image URL.
With the value set to false, Firefox will not decode JPEG XL through this preference. A website may still offer another format, such as JPEG or PNG, if its server or content logic supports format negotiation. Disabling the decoder does not delete local .jxl files. It also does not disable unrelated image formats; it is a targeted setting for JPEG XL decoding.
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Restore Firefox’s default behavior
If you want Firefox to choose the default for your installed build rather than forcing an override, reset the modified preference:
- Go to
about:config. - Search for
image.jxl.enabled. - Select the reset control beside the modified row. Depending on the Firefox interface, this appears as a curved-arrow or reset button.
Mozilla states that only preferences changed from their defaults can be reset or removed in the Configuration Editor. After resetting, the row may disappear from filtered results or display the build’s default value. A reset is different from setting false: reset lets the current Firefox channel decide, while false explicitly disables decoding.
This distinction matters because Firefox’s JPEG XL defaults have differed across channels and changed over time. Reset does not mean “turn JPEG XL off”; it means stop forcing a custom value. If the build’s default is enabled, resetting may leave JPEG XL enabled. To make sure decoding is explicitly off, set the value to false instead.
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Firefox versions, channels, and the default value
JPEG XL support has changed by Firefox channel and release. Do not assume that a setting documented for Nightly has the same default in Beta, Release, a developer build, or a downstream package. The most reliable evidence for your installation is the actual image.jxl.enabled value and whether the build contains the decoder.
| Firefox context | What Mozilla’s records establish | How to apply it |
|---|---|---|
| Firefox 153 Nightly release notes | JPEG XL image support was listed for Nightly, with the Rust-based decoder enabled by default in that channel. | Nightly users should still inspect image.jxl.enabled, because later releases can change defaults. |
| Firefox 152 Beta and Release history | Firefox began building JPEG XL code in Beta and Release builds while leaving it disabled by default at that stage. | Those historical defaults do not prove the behavior of current Beta or Release builds. |
| Firefox 157 plan announced August 24, 2026 | Mozilla engineer Timothy Nikkel wrote, “As of Firefox 157 I intend to turn JPEG XL decoding on by default on all platforms.” The statement describes an intended change, not a confirmed completed rollout. | Check your installed build’s value instead of treating the planned default as current fact. |
The historical Nightly and Firefox 152 information appears in Mozilla’s Firefox 153 developer release notes and Bug 2016688. The later plan is recorded in Mozilla’s Intent to ship: JPEG XL announcement and the related Mozilla Hacks article. As of October 13, 2026, those materials do not establish whether the intended Firefox 157 change was completed, revised, or delayed.
Keep two questions separate when interpreting this timeline. First, does the build contain JPEG XL decoder code? Second, is the preference set to permit that decoder to run? Firefox 152’s recorded Beta and Release status addressed code being built while the default stayed off; a false value does not by itself prove the decoder is absent. Conversely, setting a preference to true cannot compensate if a particular build does not include the implementation.
Firefox channels are not interchangeable evidence. Nightly is a development channel, while the historical Firefox 152 Beta and Release note records a different default at that point. A later intention to change the default is useful context, but it is not a guarantee about a specific installed version or a repackaged build. Check the preference in the Firefox installation you are actually using.
How to tell whether the decoder is actually available
The preference value answers whether decoding is enabled, but it cannot add decoder code that is absent from a particular build. A true value therefore means “allow JPEG XL decoding” only when the decoder is compiled into that Firefox package.
- Preference exists and is
true: the build exposes the JPEG XL control and is configured to allow decoding. Retry a known-good.jxlimage. - Preference exists and is
false: decoding is explicitly disabled. Toggle it totrueand retry if you want JPEG XL support. - Preference is missing: do not infer that creating it will install support. Firefox channels and downstream packages have differed in whether JPEG XL code is present. A missing row can reflect the build’s implementation or preference registration.
- Preference is
truebut images still fail: investigate the file, page delivery, and build rather than repeatedly toggling the flag.
Mozilla’s records document channel-specific behavior, not compatibility for every third-party Firefox package. A vendor, operating-system distributor, or custom build may compile different features or apply its own defaults. In particular, the absence of a row is not a reliable reason to add a new preference: the Configuration Editor can add preferences, but a new flag cannot supply decoder code that the browser package does not contain.
For a meaningful check, record the Firefox version and channel, look up the exact preference, and test with a known-good JPEG XL image. That separates a disabled preference from an image or delivery issue. If the build has no decoder, changing the Boolean will not make the test succeed; use a Firefox build whose JPEG XL support is documented rather than relying on a manually created flag.
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Troubleshoot a JPEG XL image that will not open
1. Verify the preference name
Search for image.jxl.enabled, not a similar phrase such as “jpegxl,” “jxl.enabled,” or a name with capital letters. The exact preference is case-sensitive. If the row is present, read its value rather than assuming the toggle changed.
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2. Confirm the value after changing it
Set the value to true, leave the Configuration Editor, then return and verify that it still reads true. Reload the image URL after the change. If the page serves a fallback image, inspect the actual URL or save the file to confirm that it is really a JPEG XL image and not a broken download.
3. Check the file and delivery path
A file can have a .jxl extension without containing valid JPEG XL data. Try another known-good JPEG XL file or page. A server can also send an incorrect MIME type, require authentication, or fail before Firefox receives the image. These conditions are independent of the Firefox preference.
4. Consider the Firefox channel and package
Nightly, Beta, Release, enterprise repackages, and downstream builds have not always shipped JPEG XL in the same way. Update to a supported Firefox build from your normal distribution channel, then check the preference again. Do not create a new Boolean preference merely because the row is absent; a user-created flag cannot add decoder code that was not compiled into the browser.
5. Reset the preference if behavior became unstable
If changing about:config causes crashes, rendering problems, unusual performance, or other instability, return to the row and use its reset control. Mozilla’s guidance on advanced customization is available in its Firefox advanced configuration article. If the problem continues after reset, temporarily test Firefox with extensions disabled or a fresh profile to distinguish this preference from unrelated customizations.
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Work through those checks in order rather than making multiple advanced changes at once. Start with the spelling and value because they directly answer whether the preference is configured as intended. Then test a known-good image and consider whether the installed build exposes the decoder. If you do reset the preference, confirm that it is no longer marked as a custom override; the resulting default may vary by build.
What not to do in about:config
- Do not alter unrelated image, graphics, sandbox, or security preferences while troubleshooting JPEG XL.
- Do not assume that adding
image.jxl.enabledto a build where it is absent will provide support. - Do not treat a Mozilla intent-to-ship announcement as proof that every current installation has the new default.
- Do not leave a forced value in place if you only wanted to test the browser’s normal default; reset it afterward.
Mozilla’s Configuration Editor guidance warns that advanced preference changes can have serious effects on stability, security, and performance. The JPEG XL change is narrowly scoped, but it is still an unsupported customization rather than a normal browsing setting.
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The warning is a reason to make only the change you intend and to know how to undo it. Do not experiment with similarly named preferences or create a replacement value when the expected row is missing. If you were testing whether the normal Firefox configuration supports JPEG XL, reset your override once the test is finished so later behavior follows the build’s default.
Understanding the three states you may see
State in about:config |
Meaning | Recommended action |
|---|---|---|
true |
JPEG XL decoding is allowed, provided the decoder exists in this build. | Retry the image. If it fails, check the file, server response, and package. |
false |
JPEG XL decoding is explicitly disabled. | Set true to test JPEG XL, or leave false when you need it disabled. |
| Preference absent | The build does not expose the preference in the current configuration, or its support differs from another channel or package. | Check the Firefox version and distribution. Do not create the preference without version-specific evidence that the decoder is included. |
There is also a practical distinction between an absent preference and a reset preference. An absent row does not provide evidence that support can be enabled by creating it. A reset preference, by contrast, is one Firefox has exposed and that you have returned to its default state. When Firefox’s current default changes across versions, a reset follows that build’s default rather than preserving an old explicit choice.
Does enabling JPEG XL affect other image formats?
No. The preference controls JPEG XL decoding only. It does not disable JPEG, PNG, GIF, WebP, or other formats, and it does not change how Firefox saves images. Setting it to false affects pages or files that require JPEG XL; ordinary images continue to use their own decoders.
Likewise, enabling JPEG XL does not guarantee that every website will display a .jxl image. The site must deliver the file correctly, Firefox must receive a valid image response, and the installed build must include the decoder. The preference controls one part of that chain, not the availability or validity of the image itself.
Quick decision guide
- You want to test JPEG XL: set
image.jxl.enabledtotrue, reload a known-good image, and record your Firefox version and channel. - You need JPEG XL blocked: set it to
falseand reload affected pages. - You want Firefox’s normal behavior: reset the modified preference instead of choosing a permanent Boolean value.
- The row is missing: verify the build before attempting any change; a manually added flag cannot supply missing decoder code.
- The image still fails with
true: test another file, check the network response and MIME type, and consider a different Firefox package or channel.
These actions answer different needs: true is a direct enable override, false is a direct disable override, and Reset returns control to Firefox’s default for the installed build. Choose the one that matches your goal. If your goal is simply to find out whether the browser normally supports JPEG XL, inspect the preference first and avoid forcing a value until you have a reason.
Frequently asked questions
Will changing the preference convert JPEG XL files to JPEG?
No. It only controls whether Firefox decodes JPEG XL. Use a separate image-conversion application when you need another file format.
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Usually not. Reload the image or open its URL again after changing the Boolean value. Restarting is a reasonable additional test if a page continues using a cached result.
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Is Firefox 157 definitely enabled by default?
Mozilla announced an intention to enable JPEG XL decoding by default from Firefox 157, but that announcement does not confirm the rollout. Inspect image.jxl.enabled in your installed build.
Frequently Asked Questions
What is the exact Firefox preference for JPEG XL?
The preference is image.jxl.enabled, including the periods and lower-case spelling. Search for that exact name in about:config.
What do true and false mean for image.jxl.enabled?
true permits JPEG XL decoding and false disables it, provided the Firefox build includes the decoder. The preference cannot add decoder code to a build that lacks it.
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What should I do if image.jxl.enabled is missing?
Check your Firefox version and channel. A missing preference does not prove support is available, and creating a new Boolean cannot install the decoder. Avoid adding it unless version-specific information confirms the decoder exists in your build.
How do I return to Firefox’s default JPEG XL behavior?
In about:config, search for the modified preference and use its Reset control. Reset removes the custom override and leaves the installed build’s default in effect; setting false is an explicit disable override.
Why might a .jxl image still fail when the preference is true?
The preference only permits decoding when the build contains the decoder. The file may not contain valid JPEG XL data, the page may not deliver it correctly, or the Firefox package may differ from another channel. Try a known-good image and check your build and preference value.
Was JPEG XL enabled by default in all Firefox versions?
No single default should be assumed for every release or package. Mozilla documented the decoder as enabled by default in Firefox 153 Nightly; Firefox 152 Beta and Release builds included code while leaving it disabled by default at that stage. A later Firefox 157 default-on change was announced as an intention, not established as completed in the announcement.
Can changing the preference affect JPEG, PNG, or other formats?
No. image.jxl.enabled controls JPEG XL decoding only. It does not disable other image formats or convert files.
The Bottom Line
Use about:config to set image.jxl.enabled to true to allow JPEG XL decoding or false to disable it. If you want Firefox’s own default, reset the modified preference. Check the installed version and channel, because a preference cannot provide decoder code missing from a build and Mozilla’s announced Firefox 157 default-on plan is not, by itself, proof of a completed rollout.
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