This has been an issue for a long time that has never been fixed. It's common sense that after one types their username, we should be able to hit tab to move down to the password box (much like I did logging into Mojira to submit this.)
Just a minor annoyance, but it's been there forever!
Duplicate of MC-70, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.