Using Saved Login credentials within an extension

I’m developing a private extension (at least for now) that allows the user to login against our service. The login works correctly, but I can’t seem to get the login form to remember or prompt the user to save their login credentials even though the login form and fields allow autocomplete and Firefox is currently capturing saved logins on non-extension pages. Is there a particular setting in the manifest I need to set or is this feature blocked until an extension is signed?