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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsIf Selenium stops at “Launching Firefox…”, first capture geckodriver trace logs, then verify the exact Firefox and geckodriver executables and confirm that both processes can access Selenium’s temporary profile directory. On Ubuntu Snap or Flatpak installations, the most common cause is a sandboxed filesystem that hides the generated profile from Firefox or geckodriver. A clean native Firefox installation, a clean temporary profile, and matching browser-driver confinement provide the fastest baseline.
What the “Launching Firefox…” stall means
Selenium has requested a new Firefox session, but the startup exchange has not completed. The message does not identify one specific defect: the browser may be the wrong executable, geckodriver may be undiscoverable or incompatible, Firefox may be unable to read the generated profile, or a package sandbox may place the two processes in different filesystem views.
Geckodriver is a separate WebDriver server. Selenium normally finds it through PATH, unless you configure a specific executable. Selenium’s Firefox documentation states that Selenium 4 requires Firefox 78 or newer and recommends the latest geckodriver. The exact frequency of this stall has not been established by a trustworthy published benchmark, so treat each incident as an environment-specific diagnostic problem.
1. Capture trace evidence before changing settings
Do not begin by adding random delays, disabling security features, or repeatedly reinstalling packages. Mozilla’s Firefox Source Docs says trace-level output is vital because it records WebDriver requests, protocol traffic, and Marionette messages.
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Run geckodriver directly
geckodriver -vvStart the command in a terminal, then run your Selenium test in another terminal. Preserve all output, especially the final lines before the hang.
Write logs from Selenium
In Python, configure a Firefox service with trace logging and a file destination:
from selenium import webdriver
from selenium.webdriver.firefox.service import Service
from selenium.webdriver.firefox.options import Options
options = Options()
service = Service(log_output="geckodriver.log", service_args=["--log", "trace"])
driver = webdriver.Firefox(service=service, options=options)
try:
driver.get("https://example.com")
finally:
driver.quit()If your Selenium binding exposes a different service-argument API, use the equivalent --log trace setting. In CI, always redirect the driver output to a retained artifact so a failed job does not erase the startup exchange.
Interpret the last successful step
- A failure while locating or validating Firefox points to the binary path or package launcher.
- Messages about creating, copying, or opening a profile point to permissions,
TMPDIR,--profile-root, or sandbox visibility. - A driver protocol or Marionette mismatch points to browser, geckodriver, or Selenium compatibility.
- A successful local launch followed by a CI-only hang points to display, user, container, or filesystem differences; test headless mode after the baseline works.
2. Verify which Firefox executable Selenium is using
Selenium supports selecting an alternate Firefox binary. Make the path explicit while diagnosing so a wrapper, launcher, or stale installation cannot be mistaken for the real executable.
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Find the candidate paths
which firefox
which geckodriver
readlink -f "$(which firefox)"
readlink -f "$(which geckodriver)"On Windows, use the full path to firefox.exe and geckodriver.exe; on macOS, identify the executable inside the Firefox application bundle. The important result is the actual executable, not merely a shell alias.
Set the Firefox binary explicitly in Python
from selenium import webdriver
from selenium.webdriver.firefox.options import Options
from selenium.webdriver.firefox.service import Service
options = Options()
options.binary_location = "/path/to/firefox"
service = Service(executable_path="/path/to/geckodriver",
log_output="geckodriver.log",
service_args=["--log", "trace"])
driver = webdriver.Firefox(service=service, options=options)
try:
driver.get("https://example.com")
finally:
driver.quit()Replace both paths with files that exist and are executable by the account running the test.
Special case: Ubuntu Snap
Mozilla warns that supplying /snap/bin/firefox as the binary can produce “binary is not a Firefox executable.” The Snap path is a launcher, not necessarily the confined Firefox binary geckodriver expects. If you keep Ubuntu’s Snap Firefox, use the matching confined geckodriver at /snap/bin/geckodriver and follow the package’s documented binary arrangement. Otherwise install a non-container Firefox release together with its matching geckodriver and configure both explicitly.
3. Check geckodriver discovery and version compatibility
Confirm that the driver Selenium starts is the one you intended:
geckodriver --version
firefox --versionCheck the trace log for the resolved driver path. Remove old copies earlier on PATH, or pass Service(executable_path=...) so there is no ambiguity. Keep Selenium, Firefox, and geckodriver current enough to support one another. Selenium’s current guidance recommends the latest geckodriver; its Firefox documentation requires Selenium 3.11 or newer for geckodriver and states that Selenium 4 requires Firefox 78 or newer.
Do not assume that upgrading only Selenium repairs a package mismatch. A newly installed driver can still start the wrong Firefox wrapper, and a current Firefox can still be invisible to a driver running in another confinement.
4. Treat Snap and Flatpak as filesystem problems
Containerized Firefox may see a different filesystem from geckodriver. Selenium generates a temporary profile, and Firefox can hang at startup if that profile is outside the sandbox’s accessible paths. This is a visibility problem, not a page-loading problem.
Use matching confinement
For Ubuntu Snap Firefox, use /snap/bin/geckodriver so the driver runs in the same confinement. A native Firefox installation paired with a native geckodriver is the simplest baseline. Do not mix a host geckodriver with a confined browser unless the package documentation explicitly supports that arrangement.
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Choose an accessible profile root
If the sandboxed package must remain, set TMPDIR or geckodriver’s --profile-root to a directory both processes can read and write:
mkdir -p "$HOME/selenium-tmp"
chmod 700 "$HOME/selenium-tmp"
TMPDIR="$HOME/selenium-tmp" python your_test.pyUse a directory inside the package’s permitted filesystem. The exact allowed locations depend on the Snap or Flatpak policy; consult that package’s confinement documentation rather than copying a path from another distribution.
Check permissions as the test user
id
ls -ld "$TMPDIR" /tmp
touch "$HOME/selenium-tmp/write-test"The user launching CI must be able to create, read, rename, and delete files there. A directory writable only by your interactive account can still fail under a service user.
5. Reduce profile variables
Begin with Selenium’s anonymous temporary profile. If you pass a custom profile, Selenium copies it into a new temporary directory; a large profile, lock file, extension, or inaccessible preference can hide the underlying failure.
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Baseline with no custom profile
from selenium import webdriver
from selenium.webdriver.firefox.options import Options
options = Options()
driver = webdriver.Firefox(options=options)
try:
driver.get("https://example.com")
finally:
driver.quit()If this works, add your custom profile, preferences, and extensions one at a time. Never point two simultaneous Firefox processes at the same live profile.
Use a dedicated profile only when necessary
Keep the source profile small and readable, and let Selenium copy it. Remove stale lock files from disposable test profiles. For CI, create a fresh workspace per job instead of reusing a developer’s home-directory profile.
6. Add headless mode after a normal launch works
Firefox accepts the -headless argument. Headless mode removes display-server requirements common in CI, but it cannot repair an invalid binary path, an inaccessible profile, or a driver mismatch.
from selenium import webdriver
from selenium.webdriver.firefox.options import Options
options = Options()
options.add_argument("-headless")
driver = webdriver.Firefox(options=options)
try:
driver.get("https://example.com")
finally:
driver.quit()When possible, prove a normal local launch first, then enable headless mode in the same environment. If only headless mode fails, compare the CI user, DISPLAY variables, container mounts, resource limits, and temporary-directory permissions.
Remedy comparison
| Environment | Best baseline | Profile requirement | Driver path | Logging |
|---|---|---|---|---|
| Native Firefox | Clean temporary profile | Any directory writable by the test user | PATH or explicit native executable | Retain trace output |
| Ubuntu Snap Firefox | Matching Snap geckodriver | Path visible inside the Snap confinement | /snap/bin/geckodriver and a supported Firefox binary arrangement |
Retain trace output |
| Flatpak Firefox | Matching sandbox-aware setup | Use an allowed profile root via --profile-root or TMPDIR |
Driver must see the same browser and filesystem | Retain trace output |
| CI or Docker | Fresh workspace and clean profile | Shared writable mount for both processes | Explicit paths preferred | Save logs as build artifacts |
Troubleshooting common symptoms
“binary is not a Firefox executable”
Cause: Selenium was given a launcher such as /snap/bin/firefox rather than a supported executable path. Fix: remove the override, use the package’s documented full binary with matching confined geckodriver, or install native Firefox and point Selenium to it.
Geckodriver starts but Firefox never appears
Cause: profile creation or access failed. Fix: inspect trace lines around profile creation, test a clean profile, verify the effective TMPDIR, and ensure the CI user can write there.
Works locally, hangs in Docker or CI
Cause: different user, mount, package confinement, display configuration, or resource limits. Fix: print executable paths and versions in the job, retain trace logs, use a fresh writable profile root, then add -headless.
Only a custom profile fails
Cause: locked files, extensions, oversized data, or inaccessible copied contents. Fix: return to the anonymous profile and reintroduce profile changes individually.
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Repeated retries change nothing
Cause: retries preserve the same invalid path or inaccessible directory. Fix: stop retrying, capture one complete trace, and correct the environment identified by the final successful startup step.
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FAQ
Should I delete every Firefox profile?
No. Use a disposable clean profile for diagnosis and delete only profiles you created for testing. Do not remove a personal profile before confirming its contents are backed up.
Can increasing Selenium’s timeout fix this stall?
A longer timeout can provide more time for a slow machine, but it does not fix an inaccessible profile, wrong executable, or incompatible driver. Trace the startup exchange first.
Is headless Firefox required in Docker?
No. It is often useful where no display server exists, but a normal launch is the better diagnostic baseline when available.
Frequently Asked Questions
Should I delete every Firefox profile?
No. Use a disposable clean profile for diagnosis and delete only profiles created for testing.
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A longer timeout may help a slow machine, but it cannot fix an inaccessible profile, wrong executable, or incompatible driver.
Is headless Firefox required in Docker?
No. It is useful without a display server, but first establish a working baseline with correct paths and profile access.
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