"To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier."
This alert occurs in Firefox when a form is submitted using an iframe and reload the parent page on formsuccess. if we click the continue button the the form is submitted twice.I got this problem while I used a form in ColorBox(iframe) and reloaded the parent page on ColorBox close.
Then I solved the problem using the following javascipt code instead of 'parent.window.reload();'.
var $rUrl = parent.window.location.href;
parent.window.location = $rUrl;
Note: This post is for my reference only.