When does FireFox ask for confirmation on page redirect?

Question

Every once in a while I click a link and see the following (I'm using FF 11.0):

I was wondering why FF does this on some redirects/links and not on others. I was thinking maybe it does that when you're on a page that contains a <form> element?

Thanks for any help here/

Answer 1

It's done with JavaScript, using the window.onbeforeunload event (Mozilla, IE).

Some browsers will even let you specify (part of) the text in the dialog box by returning a string from the event handler, but for security reasons, recent versions of Firefox (which you appear to be using) will ignore the string and just always present the same fixed message.

Here's some example code from the MDN page:

window.onbeforeunload = function (e) {
  e = e || window.event;

  // For IE and Firefox prior to version 4
  if (e) {
    e.returnValue = 'Any string';
  }

  // For Safari
  return 'Any string';
};