What atlef said. I don't use that many extensions in Thunderbird (unlike in Firefox =) and have shared my mails between Vista and Linux Mint from day 1 of dual booting. I haven't encountered even the slightest problem. Firefox may cause more issues because of the more exotic extensions and the fact that there isn't a 3.0 final package in the Mint repos, yet. You might install the non-native, unbranded Ubuntu package if you're keen on sharing your Firefox profile, though.
War does not determine who is right - only who is left. - Bertrand Russell