If you’re finding that your PC creeps along after a moment of quick typing in Mozilla, then you’re running into bug 188318. As far as I know, this only affects the recent nightlies and not the regular releases.
All the same, I hope it’s fixed soon. Sometimes I’ll be typing a mail message and I’ll notice that the screen is updating really slowly. Luckily, Mozilla buffers all the keystrokes (so, they're not lost) but typing while the buffer is being processed can lead to 2-second delays before a typed character appears on the screen (I timed it).