Software Update Failed, caught in a loop could not be installed, tries again, and again, and again
The first dialog box, not in a specific order is... Software Update Failed, with the message "The updae could not beinstalled. Please make sure there are no other copies of Firefox running on your computer, and then restart Firefox to try again." <Press OK>, then a dialog box called Firefox Update, with the message "Firefox is installing your updates and will start in a few moments." back to the original the first dialog box. At first it tried to do the update as a partial, then said it would do a complete, downloaded a much larger file, THEN I restarted the computer (nothing told me to) and that's when I got caught in this loop.
Opaite Mbohovái (2)
For how to troubleshoot this, see http://kb.mozillazine.org/Software_Update
Had this problem with the 3.6.14 update. Referred to the suggestion by TonyE, which was partially useful. You do need to kill firefox processes by going to Windows Task Manager (Ctrl>alt>del), but the process i had to kill was plugin-container.exe. You have to right-click it, and choose "kill process tree" to end the process. For some reason, "kill process" didn't do it for me.