mostly issues in FF, 3.57 , 3.6 or else come from add-ons or a corrupted profile. Best way to find out is to troubleshoot that in safe mode (FF safe mode) or disabling extensions one by one. If that doesn't change anything, creating a new profile generally solves everything (and sometimes reinstalling the program too, ie from scratch and not using an upgraded version anymore). And honestly, I haven't seen avast (yet) interfere negatively with FF. Clearing private data on a regular basis is a good option too.