That’s assuming that FF can’t find example.com - Rémon’s answer is correct (and he leaped on it where I wasn’t even thinking in that direction :tup: ).
Well, the other issue is that Firefox (or at least all my copies I’ve had) will by default add in a www if it attempts a url without and receives a server not found error. So if in Firefox I go to http://example.com, if that’s server not found it will immediately try http://www.example.com.
Have you tried clearing Firefox’s cache, closing it, starting it again, and then try again?
Sometimes Firefox caches the way websites should / should not redirect.
When I try the link you supplied in Firefox 3.6.8 it works as intended (i.e. it redirects to the www-domain).
Your .htaccess is also looks correct, so that shouldn’t be the problem IMO.