I'm an FF user and have not come across this problem, whenever I find things going astray I usually run trendmicro's housecall online av n mal/spyware checker to see if I've picked up some stray net nastiness and it seems to pick up things my standard av misses.
I'm not saying it's a cure-all, but it's worth giving a try - find it here:
http://housecall.trendmicro.com/