[FAQ] Troubleshooting Firefox


Almost 100% of the time, issues with Firefox are not issues with Firefox but issues with extensions or your configuration. This post sets out the basic steps to narrow down where the issue lies.


The usual steps for troubleshooting Firefox:

  1. Clear the browser cache
  2. Start in Safe Mode
  3. Start with a clean profile
    a) Start with fresh preferences
    b) Remove old security/key database files

Before each of these, make sure Firefox is not running.

Clear the browser cache

If you can’t do that from within Firefox (Preferences->Privacy and Security->Cached Web Content… Clear now), simply rm -fr $HOME/.cache/mozilla/firefox . This won’t break anything so can be done at (almost) any time.

Start in Safe Mode

From a terminal, run firefox -safe-mode . This will temporarily disable all extensions but leave your preferences intact. This means you can narrow down the issue to an extension.

If things work, disable all extensions and start Firefox normally, then re-enable the extensions one at a time until you find the one that has the issue.

Start with a clean profile

From a terminal, run firefox -P . This will allow you to create a new, clean, profile without any extensions or preference customisations. If the issue does not occur then the cause lies within something you have configured.

Start with fresh preferences

prefs.js contains preferences you changed from every version of Firefox you’ve ever run. Sometimes these can have unintended/unknown effects.

Navigate to your Firefox profile directory (somewhere under $HOME/.mozilla/firefox ) and move prefs.js to e.g. prefs.js.bak . If you start Firefox and the issue does not occur, it’s an issue within that original preferences file.

Remove old security/key database files

Occasionally, updated system or Firefox libraries can break older security and key database files.

Navigate to your Firefox profile directory and find any cert*.db and key*.db files. Starting with one or the other, rename these as a backup and start Firefox. If the issue does not reoccur, you can leave the new files in place.

If and only if all of these steps do not resolve the issue then Firefox may be at fault.



I’ve just found out that language packs can also create memory leaks under certain conditions, so if after disabling extensions you still find issues then remove any language packs too (especially any installed via AMO rather than the repos):