Its hardware acceleration, a changed about:config entry or an add on. Since it crashes on start up, try starting Firefox with a new profile from the Profile Manager. This way you can add one add on at a time and restart it with the Profile Manager to check if an add on is a culprit. Profile Manager - Create, remove or switch Firefox profiles else maybe try toggling the hardware acceleration if you have a start page that uses an extension? Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

(I assume you meant Firefox's Safe Mode instead of Windows' Safe Mode, but please update us if we misunderstood. )

Sorry to hear about the crashes. If you are getting the Mozilla Crash Reporter form, your Firefox may have logged some data that would help track down the cause of the problem. You can submit that data to Mozilla and share it with forum volunteers to see whether it points to the solution. Please check the support article "Firefox crashes - asking for support" for steps to get those crash report IDs from the about:crashes page, and then post some of the recent ones here.