Is text rendering any different? Might be that you have zoomed the page in FF (or IE)?
Can try Ctrl-0 to reset zoom sizing (works in both), I have a logitech mouse with a free spinning wheel, so this happens to me quite a bit when I forget the scroll wheel is still spinning and hit the Ctrl key. Plenty other places where the zoom etc could be set.
Could also be something entirely different, but generally I find that IE and FF render images much the same.
Wouldn't call the box fubar based on that, more likely a software setting somewhere... but hey, it's nearly Christmas, and if you want an excuse for a new box, I won't tell anyone :)