Firefox 0.10, filing under "Browser" based on bug 122445. When connecting to a site with a username or user and pass specified in the URL, Firefox presents the user with a warning explaining what will actually happen. There should be a checkbox to disable this warning in the future (defaulting to "continue to show"), for people who actually know how to read a URL. I should think some intranets make use of user@host links for legitimate purposes. In fact, I think ours used to. Firefox would be unusable for navigating such intranets, if every link popped up a warning.

although your suggested solution is a little different, I think this would be covered by bug 243572 (however that is solved) *** This bug has been marked as a duplicate of 243572 ***