That happenned to me with a recent upgrade. I did a full uninstall. Removed the Mozilla program folder, and downloaded+installed the latest version from scratch.
Then of course, it always good to have a good backup browser. Because IE sucks so much a$$, Chrome is your best bet, or Opera if you like being esoteric.
Also ensure your OS hasn't all of a sudden taken to blocking FF when launching, which can happen with certain types of malware.
Asinus asinum fricat