That popup is not needed. Firefox does this simply through auto-complete. The user starts typing their email address, and voila, the browser completes it.
This is a nice example of a feature that is trivially accessible and yet unobtrusive.
(Alternatively, you can press the down-arrow on the empty field, which will open the auto-completion as well.)
Old Opera went one step further, where it would fill forms using a Wand button. This approach also was used for logging in.
IMO, much better way, since it works well in situation where your passwords are encrypted and browser is configured to forget master key after a while.
Firefox in that scenario will bug you about master password each time you go to page where such password is stored.
