See http://kb.mozillazine.org/Software_Update (Software Update not working properly)

Remove the files in the updates and updates\0 folder:
Vista/Windows7:

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates
(%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\updates)

• Download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• Remove the Firefox program folder before installing that 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 (rei)nstall, but make sure that you do not select to remove personal data if you decide to uninstall Firefox.

Thanks! A little complicated but I got it.

Thanks for the quick reply too.

You're welcome

I have been having this problem since V3.5. Firefox is the only update I have ever downloaded from the web that needs you to uninstall the previous version – it is a brilliant piece of software otherwise. I have other things to do rather than uninstall and reinstall programmes, so I do not update Firefox anymore, but would prefer to. Why is does this problem persist?

Helpful hint for those using Windows 7 and have trouble finding the folders mentioned above:

Click start, Where it says "Search programs and files" enter %LOCALAPPDATA% and hit enter or return. A browser window pointing to C:\Users\<user>\AppData\Local should appear.

Then remove the Firefox program folder as instructed above.

I uninstalled and reinstalled firefox. Uninstalling may not be necessary. I did not lose any settings, firefox restarted just like before I uninstalled it.

I found a simple way to fix this problem ... at least it worked for me and it made sense. Navigate in windows explorer to:

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox (%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\)

and delete (or move elsewhere if you want to restore it) the file:

active-update.xml

Restart Firefox. This file has information concerning the downloaded update. For some reason 3.6.11 gets this stuck. It happened on several of my computers and removing seems to have fixed them all. I updated normally after and I'm still fixed.