When you get the page that tells you that the post time has been exceeded, do not go back to the previous page.

Click on the "Tools" menu and select "Live HTTP headers".

(You might want to do that now in order to make sure that you have this menu item. I believe that it comes with Firefox by default these days, but, if not, you might want to install the appropriate extension before you get in this situation.)

It will bring up a new window. Press reload in the browser window. It will warn you that you are about to repost data. Click OK. You will get the same timeout page, but the "Live HTTP headers" window will get populated.

Click the "Save All..." button on the "Live HTTP headers" window and save it with the filename "empegpost.txt". Then open that file from Explorer. It will open in Notepad. Within Notepad, you will find the text of your post, amongst some other stuff you can ignore. Select the text of your post and copy it.

Go back to your browser window and press the back button. The textarea where you wrote your post will, most likely, be empty. (If not, select it all and press the backspace key.) Then select the textarea and paste the text of your post copied from Notepad into it. Then click submit.

You should have recovered your post.

You can close the "Live HTTP headers" window and Notepad, and delete the "empegpost.txt" file once you've confirmed that your post has been accepted.