Do you mean the bar "Do you want to remember the password for.. on ...?" And is the problem still present with the latest version: http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-central/
Yeah, this is unfortunate but required. We listen to a special event that fires right before onformsubmit, because some pages modify/encrypt the password value in *their* onsubmit handler. I don't think there's any event for a successful form submission -- we won't know if there's an onformsubmit handler running after us that might return false. The side effect here is harmless. The user can just close the bar. If they ignore it and resubmit the form with corrected data (such that the pages onformsubmit validator allows it to proceed, as is the usual reason this stuff is done), then the old bar will be replaced by a new one with the correct login.