How do I suppress the "are you sure you want to leave this page" popup from contribution page
CiviCRM uses jQuery to watch form inputs and give a warning only if there are unsaved changes. You'll find the code for it in Common.js. When I try out a sample contribution page on the demo server it looks to be working correctly; if I open a contribution page, enter nothing and close the tab, I get no warning. But if I enter an amount and then try to leave it pops up the onbeforeunload message.

If you're up for it, debugging the CRM.utils.initialValueChanged function to find out why civi thinks the form has been changed when it hasn't would be helpful feedback for us improving the software.

Or you could just add a little javascript to your FB login button. If the existing markup looks like &lt;button type="submit"&gt; then you could add to it so it looks like &lt;button type="submit" onclick="window.onbeforeunload = null;"&gt;

But seriously, the debugging feedback would be helpful.