Sorry for necroposting, but i found the reason, its because of the Autofill plugin, i added rule to fill login form with my nickname becasue Firefox could not keep it autofilled, and it caused this.
My bad ._.
I dont exactly know how this all works, but i never experienced that stuff before
i bet it tied to some 'tags' that fields have or something
Makes sense.. auto-fill plugins commonly check the field names and try to fill in what makes most sense following a very simple "logic". You already used your name to log in which works with another form so I assume that's why the plugin knows your name. It probably thinks that it makes sense to fill in your name as soon as it spots a field called "user" or "name"...
There are tags which can be used to signalize that a field should not be auto-filled as far as I know. I could add these to that field.