After relogging just now I found tabs and appearance changed and all my security addons have gone. No Scripts, Flash Block, Add Block the lot. The only ones left are Evernote Clipper, Brit Dictionary, Garmin Communicator and the disabled Flashblock. OS XPPro Firefox Version 33.1 (which I'm about to update to 33.1.1) I tried a System restore to yesterday, but, that didn't help Note: I eventually reinstalled my security extensions and found that Firefox had remembered my preferences for them including those of Classic Theme Restorer. Next day after logging on to PC FF had again reverted to default look. This time Extensions were still there so I went to click on Options button for Classic Theme Restorer and FF froze. I had to use Ctrl + Alt + Del to exit. I then relaunched FF and updated to 33.1.1 and after relaunching I found Classic Theme Restorer had gone. All other recently reinstalled Extensions were still there. I installed Classic Theme Resorer again and after relaunch FF was back to my preferred look. I hope this is not going to happen ad infinitum. I'll now try to update troubleshooting info.