Every so often this happens and if you open the task manager and use it to end the firefox process, it will work the next time you try to run Firefox.
Friend has a Compaq desktop computer running Windows XP SP3 with 512MB of RAM. She just upgraded Firefox to the latest version. Now when she tries to start Firefix, the hour glass comes up for a few seconds but then nothing after that. Firefox never starts. Any help would be appreciated.