I am also getting these on Windows Vista SP1 also. So it is not a OS problem.I had faced some problems with ff3beta4 to be exact.the fonts look weird,in forum/type space there is a very little line splash of random colors(rainbow?) when we move mouse over it,when typing inside,there is some weird lines drawn etc(tough to explain!) BTW,I am on GNU/Linux and got a nvidia card.
with pre5 they have fixed this.seems to me a GNU/Linux/BSD problem only.