how to save username
All solutions I find on Firefox say how to save username and Password, but the solution offered is always relating to saving the password and not the username.
ప్రత్యుత్తరాలన్నీ (1)
Password Manager saves both the username and password. No option to save only the Username.
Usernames may get saved as textarea "form data" (part of search and form history), but not relative to a specific domain / sub-domain. And "form data" isn't autocompleted, it needs at least one character of keyboard input to begin the "recall" of that data.
Maybe there's a "form" add-on that does what you want.