When you say "form data" are you talking about the content of posts and PMs? I use Chrome and I've never had a problem with losing content if I hit the back button.
Yeah. Form data is lost with Firefox and IE when you hit the back button. I think this is the default behavior with HTTPS sites.
If the forum says you need to wait to post etc, it gives you a little back button hyperlink which preserves what you wrote in the form on the previous page. If you just click your browsers back button then it gets cleared.