Run the Firefox program once as Administrator (right-click: Run as Administrator).
If that doesn't help then do a clean reinstall.

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0: http://www.mozilla.com/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.

You do not need to create a new profile, that is not necessary for this issue

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.

cor-el, thanks so much for trying to help. i had in fact already tried both logging in as administrator AND uninstalling current version of firefox. anytime i try to click on anything that has to do with firefox, i get that same popup message: "Your computer must be restarted to complete a previous upgrade to Firefox. Do you want to reboot now?" can't affect it as administrator, and can't uninstall. tried to downgrade to 3.6.2, and now i have three versions of firefox, all of which are stuck on that frustrating message. i've resorted to using explorer.