gifs animate at dif speeds IE/FF

[CrazyEyez]

Some animated gifs I see move at the same speed in FF or IE. But some, like my avatar, I see move faster in FF. The thing is, I can't even tell which one is the correct speed. I've seen a few that look too fast in FF but just right in IE, and some look just right in FF but too slow in IE.

Any ideas why?