Firefox ESR has TLS support and should work on XP until next year, at least. Telling people to switch browsers to use the site isn't great, but it's certainly an easier sell than switching the OS.
I'm guessing people still running XP are running Firefox or Chrome anyways, I'm not sure if IE8 can even render most pages on the internet. That would be like trying to surf the net with Firefox 3.5 :O