In a decent browser you should be able to hit "back" and go back to
the previous page where your form will still be filled in...

Certainly can with Firefox, however with google the form will be hidden
- you'll have to click the "reply" button on the post you replied to.

Last time I used internet explorer click "back" lost everything you
typed, but as IE is so far behind on useful and essential features,
doesn't run on my personal computers, and is a security hole, I can't
be bothered checking.