hmm, sounds like an issue with FF itself. No idea what it could be. Anything similar happening on other sites?
As for using IE tab; I woudn't worry about "security" at all; people can say there are more vulnerabilities in product A then in Product B but when you realize that these people couldn't identify a vulnerability on a soft-shell mollusk, nor can they actually specify a single one, it becomes a question of wether they are simply following the mob mentality on the subject or wether they actually formed an opinion on their own.
Also- I've found that when in a discussion about IE versus FF regarding vulnerabilities, for some reason the FF advocates have this habit of quoting IE vulnerabilities from version 4,5 and 6, as if they actually matter today. (not to mention a good number of them are just quoting from elsewhere).
I switched from IE6 to Firefox 1.5 after a large number of virus issues.
I still had issues occasionally, even then, but they probably got in another way.
However, since I upgraded to Vista (and now 7) a year or so ago, I haven't run either a AV or software firewall and have yet to get any infection whatsoever. I'm apt to believe that it's more a case of IE7 being that much better then IE6... (UAC helps too).
Anyway, although I have hardly used either IE7 or IE8, I'm not sure that firefox is deserving of the security pedestal it seems to be given by right. FF was better then IE6, but IE7/IE8 and Firefox are on pretty even turf.