Peter takes issue with FireFox, but I have to defend it. I think a little modification to the ClientTarget on the page would remedy this particular situation (although it might not -- I need to test it).
More generally, FireFox is compliant to a fault while IE lets developers get away with non-standards based HTML. This was par for the course back in 1999, but is it still acceptable in 2004? We should be beyond the browser-detection nonsense of the 90s, but we're only part way there.
I feel Peter's pain, though, as I notice many websites render oddly in FireFox -- although none that my organization works with as we've started designing to the same standards as FireFox (and occassionally having to bend over backwards for IE compatibility). Personally, I don't mind finding these “FireFox Failures” -- it tells me a little about the owners of the site and their sophistication and/or attention to detail. Of course, I do end up using IE in some cases to get at the internet content (Microsoft sites in particular).
At least with .Net the whole browser detection thing has gotten better, even if IE really hasn't.