How to debug Firefox problems

Standard troubleshooting steps

Restart Firefox

Close Firefox completely: From the menu at the top of the Firefox windowbar, select File and then select the Quit menu item. Then restart your computer and start Firefox again.

Verify Firefox installation integrity

Open terminal and run this command:

rpm -qV firefox

If Firefox installation is correct you don't get any output. If there's something printed please re-install Firefox by rpm/dnf on terminal as:

sudo dnf remove firefox
sudo dnf install firefox

Run in safe mode

Safe mode can be used to start Firefox in basic safe configuration. Open terminal and:

• Make sure no Firefox instance is running. This terminal command:

pidof firefox

should not print any output as it reports PID of running Firefox instances. If you see any running instances please reboot your computer (or use kill/killall command).

• Launch Firefox in safe mode

firefox --safe-mode

Test Firefox with a new profile

If the steps above don't fix your problem, continue by following these instructions.

Make a new profile

A corrupt profile can cause various problems with Firefox. You can try to make a new profile test whether that solves the problem, and if so, copy your data (bookmarks, saved passwords, etc.) to the new profile.

Important: Before you can start the Profile Manager, Firefox must be completely closed.

1. Exit Firefox. To close Firefox, at the top of the Firefox window, select the File menu, and then select Quit.
2. In Terminal run: firefox --ProfileManager --no-remote
   • If the Profile Manager window does not open, Firefox may have been running in the background, even though it was not visible. Close all instances of Firefox or restart the computer and then try again.
3. To start the Create Profile Wizard, click Create Profile... in the Profile Manager.
4. Click Next and enter the name of the profile. Use a profile name that is descriptive, such as your personal name. This name is not exposed on the Internet.
5. To create the new profile, click Finish.
6. Click on the newly created profile in the Profile Manager and click Start Firefox.

Test to see if your problem occurs with the new profile. If it doesn't, you can copy your data to the new profile - see Recovering important data from an old profile for instructions.

If Firefox fails to start with your recent profile and says Firefox is already running you can try to delete lock from your default profile to recover it:

1. Find a default profile directory (it's usually ~/.mozilla/firefox/xxxxxx.default), you can get it at ~/.mozilla/firefox/profiles.ini file.
2. Remove 'lock' file for it (when Firefox crashes the profile may be locked).
3. Try to start Firefox again on terminal, check for any error messages.

Clear cookies and cache

Many problems with loading web pages can be resolved by clearing Firefox's cookies and cache. If the new profile helps you, you can try to clear your existing old profile:

1. Click on the sandwich menu button, select History, then select Clear Recent History... to bring up the Clear Recent History window
2. In the Time range to clear drop-down menu, select Everything
3. Click the arrow next to Details to display the list of items to clear, then select the following items:
   • Cache
   • Cookies
4. Click Clear Now.
5. Exit your browser

For more information, see Clear Recent History. [1]

Troubleshoot Add-ons / plugins

Sometimes add-ons for Firefox (Extensions, Themes or Plugins) can cause problems and you can disable them one-by-one:

1. Open the Add-ons Window by clicking the Tools menu and select Add-ons.
2. There are three tabs - Extensions, Themes, Plugins so select the one you want to test
3. Disable each extensions/theme/plugin in the list.
4. Test for your problem.

Firefox performance issues

There are two sources of possible performance issues - opened site/addon or Firefox itself. To identify hungry site/addon go to

about:performance

page and check for component which uses most of resources.

Firefox internal performance issues can be investigated by Firefox Profiler, go to

https://profiler.firefox.com/

site and enable Firefox profiling. Please share and attach the profile to bugreport (you can also anonymize it).

Check WebRender state (Hardware acceleration)

WebRender is Firefox hardware accelerated back end which uses OpenGL. It's enabled by default on Gnome/X.org and Gnome/Wayland for Intel/AMD graphics cards. You can try to switch WebRender to software backend to check if that solves your issues.

WebRedner Firefox setup

1. Go to about:support page and check Compositing field. It's WebRender for hardware/OpenGL rendering and WebRender (Software) for software one.
2. To disable WebRender hardware backend, go to about:config page and look for gfx.webrender.software preference and set it to true.
3. Restart browser, check at about:support for WebRender status.

Check Firefox X11 backend

Firefox can run with two independent Gtk+ backends / graphics servers while Wayland is a default one on some distros (Fedora, Ubuntu 20.04+). You can try to switch back to X11 to check if that helps.

On Fedora:

1. install firefox-x11 package
2. close all active firefox instances
3. run Firefox on X11 from menu or firefox-x11 in Terminal
4. go to about:support page, look for Window Protocol field. You should have x11 there.

On other distros:

1. close all active firefox instances
2. open terminal
3. run MOZ_ENABLE_WAYLAND=0 firefox
4. go to about:support page, look for Window Protocol field. You should have x11 there.

Check Firefox X11 GLX/EGL backend

Firefox can run with two independent OpenGL backends - EGL and GLX (details available here). EGL becomes default in Firefox 94 but you can switch back to GLX in case of issues.

1. Go to about:support page and check WebGL 1 Driver field. If it contains EGL_VENDOR/EGL_VERSION you're on EGL.
2. To switch back to GLX, go to about:config page, look for gfx.x11-egl.force-disabled and set them to true and restart browser.

Collect information for a bug report

Please provide those information to your bug report. [2]

• Attach Firefox troubleshooting data. Open Firefox and navigate to:

about:support

page and use Copy text to clipboard to copy/paste it. Attach the info to bugreport.

• Open terminal and run

glxinfo > gfxlog.txt 2>&1

and attach created gfxlog.txt file to the bug. It contains data about your graphics card.

• Open terminal and run

vainfo > vainfo.txt 2>&1

and attach created vainfo.txt file to the bug. It contains data about your VA-API video decoder.

• If you Firefox crashes, please go to a crash stat page by visiting this link:

about:crashes

and you should see crash reports ids. Please Submit them at Mozilla by Submit All button and paste Report IDs to the report at bugzilla.

• If you run Fedora attach exact Firefox version, type this command in terminal:

rpm -q firefox

Create screenshot/screencast for a bug report

• If applicable attach a screen cast or a picture of the issue. Options include:
  • Screencast in Gnome
  • OBS studio, instructions for installing on linux
  • More screencast info: https://www.linuxuprising.com/2020/01/4-tools-to-record-your-linux-desktop.html

Testing Mozilla binaries

Some issues can be distro specific, caused by build environment, patches and so on. It's important to try vanilla binaries from Mozilla project to negate that case.

If you get the same Firefox version from Mozilla and from distro, you can use the same profile.

• Update your distro provided Firefox to latest version and test it. It has to be the same Firefox/Thunderbird version as it's provided at Mozilla official site [Firefox https://www.mozilla.org/en-US/firefox/new/][Thunderbird https://www.thunderbird.net/en-US/]
• If you still see the issue with latest Firefox/Thunderbird version from distro, download latest [Firefox https://www.mozilla.org/en-US/firefox/new/]/[Thunderbird https://www.thunderbird.net/en-US/] release from Mozilla site directly.
• Unpack the tarball. Open terminal and switch to a directory with downloaded tarball and run:

tar xf tarball.name

• Switch to extracted directory (usually name firefox/thunderbird) and run it locally as:

./firefox

this Firefox instance uses the same profile and environment as your distro provided Firefox (unless you're running Snap/Flatpak).

You may also check clear profile and different backend (X11/Wayland):

• To run with X11 backend open terminal, run locally as below, create a new profile and test your issue:

MOZ_ENABLE_WAYLAND=0 ./firefox -ProfileManager -no-remote

• To use Wayland backend run as:

MOZ_ENABLE_WAYLAND=1 ./firefox -ProfileManager -no-remote

Testing Mozilla Nightly binaries

Some issues can be already fixed in Nightly which is a pre-release version. Please check if you can reproduce it issue there.

• Download [Firefox Nightly https://www.mozilla.org/en-US/firefox/channel/desktop/] from Mozilla site.
• Unpack the tarball. Open terminal, switch to a directory with downloaded tarball and run:

tar xf tarball.name

• You need to create a new profile for your Nightly instance. If you use Nightly with your recent (production) Firefox profile, the profile may be corrupted and you may lost your data. Switch to extracted directory (usually name firefox) and run it locally and create a new profile at profile manager:

./firefox -ProfileManager -no-remote

• To run with X11 backend run as:

MOZ_ENABLE_WAYLAND=0 ./firefox -ProfileManager -no-remote

• To use Wayland backend run as:

MOZ_ENABLE_WAYLAND=1 ./firefox -ProfileManager -no-remote

Testing Firefox titlebar

In case of unexpected Firefox titlebar behavior (missing titlebar, doubled titlebar, missing 'Title Bar' checkbox at Customize menu) try to run Firefox in terminal as:

MOZ_GTK_TITLEBAR_DECORATION=client firefox

and

MOZ_GTK_TITLEBAR_DECORATION=system firefox

and check of that helps.

Use Mozregression tool

Mozregression tool [3] can be used to find offending changeset. Install mozregression on terminal by

pip install mozregression

and run

mozregression --good version_good --bad version_bad

where version_good and version_bad are affected Firefox version. For instance when something bad happens between Firefox 78.0 and 79.0, you can find that by:

mozregression --good 78.0 --bad 79.0

(You always should use full release number like 78.0 for a regular release or 78.0.1 for a dot release).

The mozregression tool will launch various Firefox versions and as you if the launched version is good or bad. When the bi-section is finished a regression range is printed.

To test Firefox in Wayland, run mozregression with MOZ_ENABLE_WAYLAND=1 env variable, i.e.:

MOZ_ENABLE_WAYLAND=1 mozregression --good 78.0 --bad 79.0

To test Firefox in XWayland/X11, run mozregression with MOZ_ENABLE_WAYLAND=0 env variable, i.e.:

MOZ_ENABLE_WAYLAND=0 mozregression --good 78.0 --bad 79.0

If you have a specific profile where the bug happens, you can tell mozregression to use it for testing. For instance I have a Firefox profile named default and I want mozregression to use it. First I need to locate the profile files, in my case it's stored at /home/user/.mozilla/firefox/80thyv9a.default (/home/user/.mozilla/firefox is a Firefox profile directory). To test it under Wayland I run:

MOZ_ENABLE_WAYLAND=1 mozregression --profile /home/user/.mozilla/firefox/80thyv9a.default --profile-persistence clone-first --good 78.0 --bad 79.0

(note the --profile-persistence clone-first parameter to clone the profile before use which protects your data)

A different application can be run by --app parameter. To probe thunderbird run:

mozregression --app thunderbird

More info is available at Mozregression doc page.

Testing different Wayland compositor

Actual Wayland compositor may be source of various bugs. Fedora considers Mutter (Gnome) Wayland compositor as a default one. If you run Firefox on a different compositor (Sway/KWin) please also try to reproduce the bug on Mutter as it helps to sort out Firefox and compositor (Mutter/Sway/KWin) bugs.

You don't need to install whole Gnome environment for it, just install mutter package and run Mutter as a nested compositor on terminal as:

mutter --wayland --nested

Mutter will create a new Wayland session inside your active one. To run Firefox (and any other Wayland application) there you need to set proper Wayland display by WAYLAND-DISPLAY env variable. You should be already running on 'wayland-0' and the new one (created by nested compositor) may be 'wayland-1' then. So launch Firefox on nested compositor as:

WAYLAND_DISPLAY=wayland-1 MOZ_ENABLE_WAYLAND=1 firefox

It's also useful to test KDE Plasma compositor (KWin) which can be also run as a nested compositor. For testing I launch it as:

kwin_wayland --xwayland --width 1920 --height 1024

and then run Firefox as above (with wayland-1 Wayland display set).

Getting Mozilla crash report from running or frozen Firefox

You can kill running application by kill signal and then obtain and submit Mozilla crash stats.

To terminate all firefox instances run on terminal:

kill -s 11 $(pidof firefox)

This should terminate all Firefox instances and produce Mozilla crash report dialog. In next Firefox run you should see crash ID at about:crashes page. Please submit the crash to Mozilla and paste crash ID to bugreport.

Get system log after system freeze / restart

If you suffer from whole system lockups / sudden restart you can examine system log from previous session and look for relevant entries like gfx driver errors, kernel errors or Gnome/Mutter failures. Right after restart run on terminal:

journalctl -b -1

It shows a log from previous session, the '-b -1' param tells journalctl to show log from previous boot.

Additional Reading

• Additional accessibility prefs can be found here: [4]

• Additional hidden prefs can be found here: [5]

• Mozilla's own basic trouble-shooting page: [6]

• Mozilla support page: [7]

• StackTraces#Firefox