You know that cool little loading icon on the top right in the Firefox browser? Apparently it's called a throbber and you can change the link it goes to when you click on it.
In the Filter box, type browser.throbber.url (although, just typing throb should suffice).
Under Preference Name, there should be a listing for browser.throbber.url. Select the listing and then right click > modify. Change the value of the URL in the text field, hit OK and you're done.