Putting two dummy fields in between the actual username field and the actual password field worked for me, e.g.:
<input name = "Username" type="text" autocomplete="off">
<input name = "DummyUsername" type="text" style="display:none;">
<input name = "DummyPassword" type="password" style="display:none;">
<input name = "Password" type="password" autocomplete="new-password">
This prevented the autofill of the username, as well as the autofill of the password.
autocomplete="new-password" is apparently not fully implemented, but can help with some browsers. I doubt that it was helpful with FireFox, but it is there for future versions in case the feature is more widely adopted.
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