Standard diagnostic

If Thunderbird, Firefox, or Mozilla Suite won't start or is no longer working properly and no other application is exhibiting strange behaviour, follow these steps.

1. Reboot the computer. If the problem persists:
2. (Firefox and Mozilla Suite): If you can open the program, clear the cache: "Tools -> Options -> Privacy -> Cache -> Clear" (Firefox) or "Edit -> Preferences -> Advanced -> Cache -> Clear Cache" (Mozilla Suite). If the problem persists, continue on.
3. Close the program (if open) using "File -> Exit" and make sure it is not running in the background. Do not skip this step.
4. (Firefox and Thunderbird only) Start the application in Safe Mode (not Windows Safe Mode). This will start the program with all extensions disabled and the default theme. If the problem goes away in Safe Mode, then select the default theme in "Tools -> Themes" and use "Tools -> Extensions" to uninstall your 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, try uninstalling that one first, then restart Firefox or Thunderbird. If the problem persists in Safe Mode, or if Firefox or Thunderbird doesn't start, even in Safe Mode, continue on.
5. Back up your profile. Your profile contains your bookmarks, passwords, cookies and other browsing data.
6. Perform step 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 step 3 again, then delete "localstore.rdf" from your profile folder. You will lose any changes you have made to the toolbars. If the problem persists, continue on.
8. Perform step 3 again, then delete the "chrome" and "extensions" folders in your profile folder. You will lose all your extensions and themes. If the problem persists, continue on.
9. Perform step 3 again, then create a new profile. If the problem is fixed, you had a corrupt profile. If the problem persists, continue on.
10. (Firefox only, Windows OS) Rename firefox.exe to firefox1.exe in the program files folder and then run (double-click) firefox1.exe. If the problem persists, rename it back to firefox.exe and continue on.
11. Uninstall the program, clear your OS temporary folder, download it again, and reinstall.

If none of these steps help, the problem could be with other running programs such as a Firewall, mail-screener, spamblocker, parental control or similar program that is conflicting with your Mozilla program. Try disabling these other programs one at a time, until you find the culprit. Also check your system for viruses and other malware.