See:

• http://kb.mozillazine.org/Browser_will_not_start_up#XULRunner_error_after_an_update

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version.
• Do not remove personal data when you uninstall the current version.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

I have a similar problem -- Firefox 5.0 (under Win XP sp3 fully patched) runs fine on one account on the machine. It fails with the XUL Runner error described in the report above on another account on the same machine.

I have followed the directions suggested -- uninstall, remove the program files/firefox directory, reinstall -- twice, with no effect.

Turning off ZoneAlarm has no effect -- though it would be odd for it to affect one account and not the other.

Also, trying to run FireFox in safe mode does not help.

It would seem that there is a subtle difference between the two accounts -- any suggestions on where to look? What is the name of the file that might have a bad copy of XUL Runner in it?

Try to install Firefox via a custom install in another directory.