When a user loads a login page for which the password manager is remembering multiple username/password combos (a.k.a. credentials), Firefox should display an credential chooser (most likely in the same style and placement as the popup blocker bar) which lists the remembered usernames and prompts the user to select one. When the user does so, Firefox should fill in the login form with that username/password combo. Optionally, if there's a way to determine to a high degree of accuracy that the login form is complete, Firefox might automatically submit the form. Otherwise it might provide additional UI for doing so, if such UI doesn't overcomplicate the chooser, since then users won't have to go to the added effort of hunting for the exact location of the login form on the page and then submitting it themselves. Optionally, the chooser might include UI for deleting credentials.

See also bug 222653 asking for the autocomplete widget to be automatically shown on focus, rather than on downarrow.

I'd say not quite the same UI, since the yellow warning look and feel isn't quite right for this. I need to fix the browsermessage API to allow a lot more flexibility before we can do this.

I feel that rather than showing up in the pop-up blocker area, this should have a similar UI to autocomplete. i.e. when the user clicks on a username textbox that has saved passwords, it should show a dropdown box with the relevent options.

I'm not sure of the need to create special UI around this, as I think the use case will diminish as popular OSes make it easier for multi-user profiles at the OS-level (ie: XP, Vista and OSX) which decrease the liklihood of >1 user making use of a single profile. I think the better solution would be the lightweight one proposed in bug 222653, albeit with some modifications (like, there should be some indication before the field is given focus that there is autofill info available).