I've run across an interesting bug in Firefox recently, but it's unfortunately the kind of problem that is incredibly difficult to track down. The header image here at this website wasn't showing up in my main Firefox profile on my PC. Other profiles worked, Internet Explorer worked, and the installation on my laptop also worked without a hitch. Trying to view the actual file in the problematic install (by entering the exact URL to the image in the address bar) only resulted in a "File Not Found" error page.
Doing a Bugzilla bug search, I ran across bug #312908, which seems to reference a similar issue. The comment in the bug reports that clearing the cache manually, then reloading the page, causes the problem to go away. Sure enough, that worked like a charm for me. Is this simply a cache problem? A more subtle rendering engine problem? It's hard to tell. Trying to reproduce the problem should be interesting.
If you run into a website that all of a sudden doesn't render properly, and you know the site isn't at fault, clear your cache and try again. Hopefully this is a bug that will get squashed as we head towards Firefox 2.0.