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 this pop-up about resending POST data is not to use the Back button, but to open links on a page requested from a server by sending a form with POST date in a new tab (window) with a middle-click or hold down the Ctrl key and left-click the link. You can close that tab or window instead of using the Back button.

I thank you, cor-el, for your reply, but I can't say I understand it.

"Firefox doesn't know what that form data means." What does this sentence mean?

I asked three questions, and it seems you overlooked the second and third: "What reason would I have not to Resend. If it's as it seems a security warning, what is the security issue?"