Greetings!
I have two Windows 8 64-bit installations with Avast 8 free and Online Armor (OA) free and this problem:
Flash (e.g., http://www.speakeasy.net/speedtest) freezes Firefox unless Avast Behavior Shield (BS) has “Monitor the system for unauthorized modifications” unchecked or OA is shut down.
Of course, I would prefer to fully enable BS.
No combinations of exclusions in Avast (Global or BS or anything) and/or OA help.
Nothing is in the logs.
???