Now paste the link into the Firefox browser address bar.
First you will prompted to log into the Administrator login. After logging in you will be redirected to the edit users page. Notice how the Username field will have your username entered.
It's a little wonky but I did produce it on firefox once. I think it's related to when the acp asks you to login, otherwise if it remembers your login details (and doesn't prompt you to enter them), it doesn't seem to trigger.