suggested usernames that are not in my list of PW's retained by Firefox ???
On repeated occasions, I have been offered multiple usernames in logging onto a website, specifically some choices that are all numeric usernames in addition to the correct one. When I then attempt to edit my Firefox PW list, there is no such username listed. I'll grant that I'm not always disciplined when offered "Save this PW and username?" at time of logins, but even if an inadvertent save, they should be there when I access the list.
(Windows 10 Browser)
That could be the form filler built into Firefox. Click in the field, enter a character or two and STOP ! Using the arrow keys, highlight an unwanted entry. Then press Shift + Delete
Repeat as needed.