You get this if you refresh or go back to a page that has been requested from the server by sending POST data from a possibly hidden form.
Are you using a bookmark or do you get this when you when yo use the Back button or otherwise?
You get an alert about resending POST data if you go back to a page or refresh a page that was previously requested from the server by submitting form data via a (hidden) 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.