Can you try to start Firefox in Safe Mode to see if it opens?

• Windows: Hold the SHIFT key while double clicking the Firefox icon on your Desktop
• Mac: Hold the OPTION key while double clicking Firefox in your Applications folder
• Linux: Run the firefox -safe-mode command in your Terminal/Konsole (Ubuntu/Kubuntu)

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

You can rename (or delete) the prefs.js file and a possible user.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Let all cookies expire when Firefox is closed.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: Keep until: "I close Firefox"

Create a cookie 'allow' exception for cookies that you would like to keep.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

Thanks for you replies! I got it running again but the problem repeated after I closed it then tried to re-open it. In the end I would up uninstalling and re-installing. My version was old, anyway. :)

Did you disable the "Clear history when Firefox closes" setting?

Yes, it's back to "remember history".