If you have tweaked your about:config page to speed your browser up, or simply installed an add-on that messed up your configuration, Firefox 3 has a simple method to restore all default settings back without uninstalling the software.

Here’s how:

1. Open Firefox in Safe Mode, from your programs menu or by typing “firefox -safe-mode” in Start->Run.

   

    

2. Choose what you want to be brought back to its original settings by checking one of the following options:
    
   • Disable all add-ons: Disables all added themes and extensions.
   • Reset toolbars and controls: Removes any made toolbar customization.
   • Reset bookmarks to Firefox defaults: Replaces your bookmarks with the default bookmarks.
   • Reset all user preferences to Firefox defaults: Restores the default Options and preference settings and resets the default theme. It also changes back to default all the entries in the about:config page.
   • Restore default search engines: Adds back all of the default search engines (Google, Yahoo, eBay, Creative Commons, Answers.com, and Amazon.com) , without removing any added search engines.

Now everything should be back to default, as if you just installed a new fresh copy of Firefox.

If you get Ajax Error Messages when using Firefox :-
Besure to check that Mozilla firefox has javascript enabled otherwise Ajax will give errors
gotomozilla-> tools->options->contents then enable javascript and allow scripts.