Rated 4 out of 5 stars

A minor point: a better name for the add-on might be:

  Password integrator for Keychain Services

References:
http://developer.apple.com/documentation/Security/Conceptual/keychainServConcepts/01introduction/introduction.html
https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsILoginManagerStorage
https://addons.mozilla.org/en-US/firefox/search?q=keychain&cat=all
https://bugzilla.mozilla.org/show_bug.cgi?id=106400#c65

Nice work, thanks!

This review is for a previous version of the add-on (1.0a6).