If a reboot is needed then that usually means that some (security) software is blocking files or you need elevated permissions.
Run the Firefox program once as Administrator (right-click: Run as Administrator).

If that doesn't help then do a clean reinstall.

• Download a fresh Firefox copy and save the file to the desktop.
• 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.

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 am running it as the administrator. I am also getting the same error message when I try to uninstall Fire fox. It won't let me uninstall when I try to preform the uninstall the same message appears, "Your computer must be restarted to complete a previous upgrade of Firefox."

Do you have any other recommendation to fix my problem?

I had the same problem and went round and round in circles trying to uninstall FF5. Nothing worked. I eventually downloaded a free "Uninstaller" programme, i.e. Revo which did the job and I have since downloaded a fresh Firefox 5 which is working perfectly.

Thank you! Thank you! Thank you!! I had never heard of Revo, but googled it and downloaded the program easily. I did a Forced Uninstall; I was a bit intimidated in the second step with the registries (which I skipped), and the 3rd step I deleted everything that was Firefox, but nothing Windows. In my searching I found a suggestion to back up Firefox with MozBackup, so I did that before the Forced Uninstall. When I reinstalled FF 3.6 (I don't like change :), everything was exactly as I had left it. Including the windows and tabs...nothing was changed. I don't know if the MozBackup did that or what. But I cannot thank you enough for posting. You saved me $$$, time and more frustration!