Firefox has some useful features for auto-completing forms, and has special handling for forms with password fields. It would be useful if it could do the same for proper HTTP authentication, allowing me to set a username and password that should be used when a site (or subdirectory of a site) requests authentication, always refuse authentication from that site, get more information about what sort of authentication it supports (Basic or Digest), and so on.
Firefox actually has partial support for this now, but it's not as fine-grained as I'd like.