Exactly what happens when you try to start Firefox? Does it open but just not connect to websites? That could be caused by your security software's firewall, especially if you just updated Firefox. See Fix problems connecting to websites after updating Firefox.

If that's not the problem, try some Troubleshoot and diagnose Firefox problems. See if you can start in Firefox's Diagnose Firefox issues using Troubleshoot Mode by pressing the Shift key while you double-click the Firefox icon ... then select "Continue in Safe Mode" when the Firefox Safe Mode window appears. If Firefox starts in Safe Mode, follow the steps in Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems to see if you can find the source of the problem.

If Firefox still does not start, either post back with more information or try the suggestions here:

http://kb.mozillazine.org/Browser_will_not_start_up

Thanks for your help. I disabled the USE HARDWARE.ACCELERATION button. Seems to have fixed the problem

Hey, that's great! I'll mark my reply above as solving the problem. If the problem returns and you need more help, post back.