I've tried several times to upgrade to the newest version of FF. Unfortunately, any 4.x version so far will instantly crash when started. Using FF Safe Mode works fine, but making all of the options when launching Safe Mode permanent, and then launching FF in normal mode still crashes it. I've tried upgrading from 3.6.x, uninstalling FF3.6.x, and then installing 4.x, and even manually removing any and all files, folders, and registry entries related to FF before installing FF4.x, but nothing works. Crash-id's I no longer have, due to repeated uninstalling/reinstalling Troubleshooting info is provided, but will most likely not be relevant, due to running FF3.6.17 at the moment In the crashlogs for the FF4.x crashes, crashadres was 0x0, reason was "EXCEPTION_ACCESS_VIOLATION_READ"