What do you mean by appears way oversized? I just checked in IE7 and Firefox 3 (3.0.8) and they appear to be the same size. Did you by chance hit CTRL ++ in Firefox to increase the size (or View -> Zoom -> Zoom in) and it is remembering that resize?
You got it in one, lol. Thanks.
I upgraded Firefox recently and that's when the problem started. I wasn't even aware of the zoom tool!
I shall have to investigate things in a bit more depth in future.