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 from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• 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.

I have this unlocatable license problem as well. The 2nd step of this solution says to "uninstall your current firefox version..." but, I am also NOT able to uninstall firefox. An error message appears stating "An Error occurred while trying to uninstall Mozilla Firefox (3.6.3). It may already have been uninstalled." But, it's not uninstalled and runs on my machine. How to correct this situation to be able to execute the other steps of this solution?

Remove the Firefox program folder (C:\Program Files\Mozilla Firefox\) manually if there are still files in that folder.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode