Firefox’s site-isolation architecture is called Fission. It uses operating-system process boundaries to make it harder for a malicious or compromised site to reach another site’s private data. The rollout is not a single, current “coming soon” event: Mozilla’s desktop announcement dates to 2021, and its April 2026 Android update says isolation was still off in Android Release and only partly enabled in Nightly.
What is Firefox site isolation?
Fission is Firefox’s site-isolation architecture. In broad terms, it places sites in separate operating-system processes so that a problem affecting one site is less likely to expose data belonging to another. Mozilla described the goal as making it “even harder for malicious sites to read another site’s secret or private data” in its May 18, 2021 announcement.
This is a containment boundary, not a guarantee that cross-site access is impossible. A site-isolation boundary works alongside other browser protections, including the same-origin policy; it does not make every vulnerability harmless.
Is Fission enabled in Firefox?
There is no single answer for every Firefox platform and channel. “Fission architecture is present” and “site isolation is enabled” are different states, and Mozilla’s published information describes different statuses for desktop and Android.
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| Platform and channel | Status supported by Mozilla’s published information |
|---|---|
| Desktop Firefox | Mozilla’s May 2021 announcement said Fission was being finalized for a subset of Nightly and Beta users, with broader rollout planned later that year. That dated announcement does not establish the setting or behavior in a current desktop release. |
| Android Release | A Mozilla employee post from April 2026 says the Fission architecture had been present since Release 147, but site isolation was off in the Release channel. The post did not give a date for enabling it by default. |
| Android Nightly | The April 2026 post says Nightly had partial site isolation; it does not describe full isolation as enabled. |
For Android’s channel-specific status, see the April 2026 Mozilla Connect post. The desktop rollout details above are historical, not a current rollout forecast.
Does site isolation stop malicious websites from reading my data?
It is intended to make that harder by limiting what a site process can reach, but it cannot promise that every cross-site flaw is blocked. Mozilla’s Firefox 154 security advisory, dated August 18, 2026, lists multiple site-isolation issues fixed in that release, including a high-impact issue. That is a reminder that isolation is a security feature whose implementation still needs ongoing fixes—not evidence that the protection has no value.
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How is site isolation different from tracking protection?
Fission is about process-level containment of web content: reducing the extent to which one site can reach another site’s data if something goes wrong. Tracking protection addresses unwanted tracking, while cookie partitioning, containers, anti-fingerprinting features, and VPNs serve different purposes. They are not substitutes for Fission, and Fission is not a tracker blocker or a privacy service.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why did Mozilla say site isolation was “coming”?
Mozilla’s May 2021 desktop post presented Fission as a rollout in progress: it was being finalized for some Nightly and Beta users, and a wider rollout was planned later that year. The post also gave those Nightly users historical steps: open about:preferences#experimental, enable “Fission (Site Isolation),” and restart Firefox. Those instructions describe the 2021 announcement and should not be treated as a verified path in current Firefox.
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The Android picture is separately qualified. Mozilla’s April 2026 employee post distinguishes the architecture, present since Android Release 147, from the isolation feature, which was off in Release and partial in Nightly. It states no default-on schedule. The two announcements describe different platforms and dates; neither supports treating “coming to Firefox” as a current, universal launch notice.
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