Ideas, yes. FF (though improving) is somewhat notorious for not doing repetitive mathematical transitions (be they fades, positional, etc.) smoothly. This is especially true when more than one thing on the page is transitioning at the same time. This is due to a relatively poor interface between the FF browser and a given operating system (in this case whatever Mac OS you are testing on).
FF 3.03 (latest release I'm aware of) is the best so far at this sort of thing, so if you are using an earlier version, upgrade. Also, if it is a case of more than one thing transitioning mathematically at the same time, try to arrange things so that these transitions can occur one after the other, rather than simultaneously.
To be more specific, I would need to see a demo page where you are having this problem:
Please post a link to the page on your site that contains the problematic code so we can check it out.
It's always possible (given that I know none of the specifics in this case), that there could be other issues at play, either in addition to what I've outlined, or without regard to it.
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