Standard diagnostic

If your Firefox/Thunderbird/Seamonkey 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 disable all your extensions in "Tools -> Extensions". Restart Firefox/Thunderbird. If the problem exists in Safe Mode or was not solved by switching themes and disabling extensions, 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.