It is not the expected behaviour, but the early versions of Firefox were quite buggy so it is not entirely surprising that you might have problems.
It sounds like a javascript error of some kind, but the error may not be in the slideshow script, it could be some other script on your page, often one script error will cause other scripts to stop running. Do you see any error message on the error console? If so, what?
Fortunately I think that the number of people who would be using Firefox 3 on those two operating systems is probably very low.
However I will check it out myself when I can, we do keep an old Windows XP with Firefox 3 for testing purposes.