In FF there is no drop in fps at all when i press f11 to full screen it. Only in chrome. Let's take the fountain example - it scales to fit whole window and for most of the time it stays at 60fps. press f11 to add those 50px that is occupied by browser ui and frame drops to around 40 on chrome 22 and to around 45 on chrome 23.
i5 sandy 2500
60gb ssd for system and programs
gt560 non ti