I've been running 64bit Firefox since version 9 and now I'm on 12 alpha 64bit and also use 64bit IE 9.

While they perform fine, they do have some variant issues at times, specially in alphas, but then I'm using it for those challenges.

Would suggest if you do use a nightly at http://nightly.mozilla.org that you also join the nightly news mailing list @ https://mail.mozilla.org/listinfo/nightly-testers.

Great source for problem solving, issues tracker and tips.