I would bet money that this has to do with FireFox 4.0. But I can't fix it.
Open the pref window (tools->options->advanced->pref editor), in the search box enter webmail.UserAgent, If you click on the key this is the value Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b4) Gecko/20090423 Firefox/3.6 GTB5 It's looking for FireFox 3.6 not FireFox 4.0 right???? I've tried changing it but no luck.. Ugh!
