I had this problem and it was a nightmare trying to upgrade FF. Looking here on the FF help forums I found this and it actually helped:

1. Go to: "C:\Program Files\Mozilla Firefox" 2. Deleted all files that end with "upgrade" and/or "delete" on the name.

Now you should be able to Uninstall FF and install any version that you want.

Hope it helps.

The solution is usually to perform a clear reinstall.

See also:

• http://kb.mozillazine.org/Software_Update (Software Update not working properly)
• How to fix the Update Failed error message when updating Firefox

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data if you uninstall the current version.
• 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 (not in the Firefox program 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.