Did you ever install a Firefox 4 beta version or a Firefox 4 release candidate (RC)?

If you did then it is possible that you are still on the beta update channel.

If you have problems with updating then easiest is to download the full version and uninstall the currently installed version.
You may need to remove the Firefox program folder to do a clean install of the new version.

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0.x: 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.
• Do not 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 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.

Just like when you bring your car in to be fixed, that squeak won't squeak - the update problem hasn't reoccurred. Thanks, though. I'll try your fix if it happens again.