Whoa, that's so out of date. I doubt anyone would be using it in the real world, almost no one. I'm not even sure you can get Firefox that old any longer. It's a case of, if someone wants to run that browser, they have to expect things like that, and much worse on a modern web page. The Fox is up to v20 or higher now. They have been adding major version numbers more rapidly lately for less changes with each update. But great leaps forward in compliant rendering came at v7 back when updates came out only once a year or less.
evolt.org or some other repository of old browsers might have a Firefox that old. If so, I could install it on my vm, but it still might not let me see what you're seeing, it might be a setting or something peculiar to your installation of that version of Firefox. Even if I could see it, the diagnostic tools of that version of Firefox were very limited, so it might be hard to determine the cause. I think service pack 3 is the most recent update to XP, but there are tons of other important updates since then that are not service packs, that might impact this. Is the XP fully up to date? Mine is. Considering all that, and the fact that virtually no one will be using that browser to view the site, and the fact that it doesn't detract from the site's accessibility or functionality in that browser, well I think you can see it's hardly worth the trouble other than as a point of academic interest.
Edit: Hmm, it appears that Firefox 3 is still supported as a separate browser from the mainstream current versions and is up to 3.6.28 or higher.
So I think I will give it a shot to see what I can see.