I too had the issue with Firefox. I discovered a workaround that can allow your rotational frames to still work in either IE or Firefox.
In an ideal situation, your site should appear the same on either browser. Using the workaround, I think we got ours to be identical on both IE and Firefox. Check it out using both different browsers here:
http://forums.hearts4horses.com.
I was able to do this by simply changing to animated gif files. This is not hard, but it does take a little time. No issues so far...