Yesterday, I updated Avast Internet Security via AvastUI from Avast 9.0.2021 to Avast 2015 10.0.2206 on both my machines. I had Avast Online Security installed but disabled in all profiles. The crashing is consistent; close Firefox clicking the x at the top right of the program window, and a minute or so later Firefox crash reporter will come up.
I have uninstalled AOS on this Toshiba via the Uninstall procedure, waited a short while and then restarted the machine in the hope that this would fix the problem. No such luck. I haven't yet done a clean uninstall and then reinstalling, in case Avast Team might want any information from my installs.