I think that IE imposes a set minimum on delay between frames, so that even if you have a delay of 1/10th of a second, it will still display at a lower framerate (I think that it bumps the delays to 7/10ths of a second or something like that). I'm guessing that Firefox doesn't have that restriction, so gifs where the delay was left really low because it looked fine in IE end up running a bit fast in Firefox.