Re: The last posting from your IP was less than 20 seconds ago. Please try again lat
The best solution is probably to not force users to hit the back button in the first place - instead display the usual comment/PM submission form with their comment already filled in and a note at the top telling them they need to wait.
That would be better, but that's a bigger change.
Not on my Firefox. I still have the form data when I hit the back button. 
That's strange. I use the latest version of Firefox. It happens even when JavaScript is disabled, and I don't think the forum is sending any cache-control headers or anything.