You get an alert about resending POST data if you go back to a page that was previously requested from the server by submitting form data via a POST form.
Firefox can only make sure to get the same page by resending that POST form.
Firefox doesn't know what that form data means, so Firefox asks for confirmation before resending that form data as such an action can cause you to repeat an action and buy another item or post a message another time.
A way to prevent that pop-up is not to use the Back button, but to open links from a page that was requested from a server by sending a POST form in a new tab (window) with a middle-click or a Ctrl + left-click.
Then you can close that tab or window to go back.

Online merchants create a unique order number. Trying to resend it will generate an error message these days, not a duplicate (we're talking about the post confirmation part, not the click to finalize CC; if you went back in that case, you'd just go back to the CC info.) No modern website that's selling anything has any sort of issue with you resending POST data (correct me if i'm wrong).

This nag is so annoying that I completely uninstalled Firefox, not even waiting till I dl'd a replacement (doing that with IE, which for all its flaws, hackability and uncoolness, at least gets the job done without changing shortcuts -i'm looking at you there, Opera- or requiring add-ons before I can configure ANYthing I seem to want to configure (firefox, obviously)).

As for the workarounds? The browser.sessionstore.postdata about:config setting doesn't solve the problem. And I'm not installing a program to click boxes for me THAT SHOULD NOT BE THERE IN THE FIRST PLACE.

Googling this issue, the complaints seem to go back to at least 2008! If you can't or won't listen to feedback for THREE years, you might as well team up with the people in Redmond.