“For several months, the Vista PC in my home office refused to upgrade
Firefox. The first few times this happened, I uninstalled the old version of Firefox and reinstalled the new version. (Instructions for installing Firefox are provided on the Firefox support site.)
Though this allowed me to upgrade the browser, the uninstall/reinstall process took far too long. The Firefox error message instructed me to “make sure there are no other copies of Firefox running on your computer, and then restart Firefox to try again.” I closed Firefox, opened the Windows Task Manager Processes list (press Ctrl+Alt+Delete, choose Start Task Manager, and click the Processes tab).
Sure enough, there was an entry for “firefox.exe *32,” even though there were no Firefox windows open on the system. I selected the rogue entry and chose End Process to close it. With the phantom process gone, Firefox updated without a hitch.
Here are solutions to four other common Firefox glitches…”
Source: Fixes for five common Firefox problems | Workers’ Edge – CNET News