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.
The only way that Firefox can make sure to get the same page is 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.
The only 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.

It is ok that we get an alert before resending post data. But we don't want resend anything when we click on back button. We just want to see the page as was shown last time. When we want to resend data we may hit refresh on the page.

That's BS cor-el! I go to a site regulary and don't get the "resend" then randomly, I sometimes do.