Provide workaround for Firefox not triggering the onchange event when prefill occurs

This is my last issue I'll submit (for today) and for this one I have no solution. Since 2001 Firefox has had a bug "onchange handler not firing when prefill occurs which affects Compact Forms used for user login forms. If you use Firefox and have more than 1 user/password saved then your form will not be prepopulated. Start typing your username and the autocomplete options appear. Then if for some reason you are inclined to use your mouse to select the user (not sure why you wouldn't use the keyboard) Firefox prefills the password but does not trigger an event, and therefore the compact form label is still visible.

The best solution I can come up with is to wait until Firefox 4 lands and #669878: Support for HTML5 placeholder attribute gets in, otherwise we'd need some really awful timer based check. Any other thoughts?