I have apache-tomcat servlet engines sitting on different ports, one is a test server, one a production server. If I go to the equivalent form on the different ports, autofill fills in the last value entered on either port, not the last value for the current port. Not only an annoyance, potentially a security risk - viz a form on :443 versus the identical form on :80 ?
And if you go to a completely different form from a completely different server and that form asks for your address, form manager (a.k.a. autofill) will use the address that you entered previously. That by design is the intent of autofill -- to build up a wallet of information from the various forms that you visit and reuse that information on new forms that you encounter so you don't have to retype the same values over and over as you surf the web.
Just realized that perhaps you were talking about single signon (a.k.a. password manager) and not autofill (a.k.a. form manager). In that case this would be a dup of bug 57923 which by coincidence was filed just a few hours after this one.