FWIW, when working with Firefox there are still a lot of format problems. I've gotten my best results by right-clicking on the Reply (or Reply with Quote) button then clicking open in new Tab, or new Window. Close the older tab or window when you finish your reply.

I'm hoping they'll be able to straighten out the formatting problems for Firefox eventually, but if not I'll just have to live with the problems. I won't use IE unless it's absolutely necessary.