This is the keyboard buffer playing bad
Either the buffer in hardware or a software buffer, which probably should be flushed
I had this problem, not as bad by far, some time ago but I can't remember what I did to solve it - something fairly simple
Oh, turn off Compiz (Appearance > Visual Effects tab > None - to get i back select the "old" option)
Boot from the live CD and see what happens, if it's software the live CD should be OK
If it's only in Firefox the solution at the very end of this topic
I'll be thinking of it but you needed a solution days ago.....