Highlight the username you want to delete by hovering curser over it (don't click) then simply press 'delete' on your keyboard. Works for me !

Highlight the unwanted username by hovering the curser over it (don't click)... Then press 'delete' on the keyboard... Works for me!

On Mac you need to use Shift+Delete.

Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry

• http://kb.mozillazine.org/Deleting_autocomplete_entries

Thank you