Hi Maanton,
Firefox uses Gecko's rendering engine for animated gif's.
This is, as you have seen different to IE.
I'm afraid there is nothing you can do about it, as it is a browser difference (some call it a FF bug), not a problem with your gif.
You could try finding a comfortable fps for your gif that comprimises between the two browsers, but if it were me, I would be happy in the knowledge that 90% of the web use IE, and the ones that don't are probably used to the issue. (I think Nutscrape and Mozilla will have the same issue as Firefox).
Cheers
Aaron