Standard diagnostic

If your Firefox/Thunderbird/Mozilla Suite was working fine but isn't anymore, follow these steps.

1. Reboot your computer. If the problem persists, continue on.
2. Completely close the problem application (using "File -> Exit").
3. Make sure the application is not running in the background.
4. (Firefox and Thunderbird only) Start the application in Safe Mode (not Windows Safe Mode). If the problem doesn't exist in Safe Mode, the problem is most probably with one of your themes or extensions. Select the default theme in "Tools -> Themes" and use "Tools -> Extensions" to uninstall extensions (although the extensions appear grayed out, you will still be able to uninstall them). If the problem occurred after installing or updating a particular extension, you can try first just uninstalling that one. Restart Firefox/Thunderbird. If the problem exists in Safe Mode, or the problem was not solved by switching themes and uninstalling extensions, or Firefox or Thunderbird didn't start at all in Safe Mode, then continue on.
5. Back up your profile.
6. Perform steps 2 and 3 again, then delete "XUL.mfl" (the name may differ slightly, e.g. "xul.mfasl") from your profile folder. This file will be regenerated on next startup. If the problem persists, continue on.
7. Perform steps 2 and 3 again, then delete "chrome" and "extensions" folders in your profile folder. You will lose all your extensions and themes. If the problem persists, continue on.
8. Perform steps 2 and 3 again, then delete the "Cache" folder in your profile folder. You will lose your cache. If the problem persists, continue on.
9. Perform steps 2 and 3 again, then create a new profile. If the problem is fixed, you had a corrupt profile. If the problem persists, continue on.
10. Uninstall the application, download it again, and reinstall.