Into the form that came up after clicking "Join MusicBrainz wiki" I entered "mmirG" as a username and a password of upper and lower case letters.
Into the "Authentication Required" box I entered "mmirG" as the username and
I just retested using "MaryMary" as username and "lamblamb" as joining password.
Then for "Authentication Required", I enterd MaryMary as user name and tried both;
Same outcome of 0.5 second disappearance "Authentication Required" box and then re-appearance of it blank.
This recent test is on a separate computer to the one from last night.
Just now - Firefox 46.0.1
18 hours ago - Firefox 38.8.0 (esr update channel)