I'm not entirely sure this is a problem with avast!. If you've installed memory, i suggest you check up memory first, because it can make Windows do funky stuff when it's not running properly.
Try running MEMTEST (basic test, no need for thorough one) and see how it goes. You may also want to fire up ORTHOS tool and run the memory test. Just to be sure.
Firefox hanging in a memory is sort of a known issue and happens to quite a lot of users that don't use avast!. Not sure why but i know it happens. Though not with the latest builds of Firefox, at least not for me.