I've got over 5 subdomains, and each with multiple logins, so it can be chaos when I try to login to each one of them, because I get the login info of all the subdomains, some have same user and different passwords etc.
So this is a beautiful solution, kudos to the firefox and mozilla team:
if you want to go back to the old behaviour, you can you can change the signon.includeOtherSubdomainsInLookup preference in about:config to false.
https://bugzilla.mozilla.org/show_bug.cgi?id=1601558