I want to chime in because I have seen this problem. I don't think it's because of the game, but rather a Godot and/or Firefox issue. I've experienced it across many games.
It is difficult to search for and this may be the first time I've seen someone else describe it, but it's as if every mouse movement turns on and off a slow drift to the right. Move it a bit and it slowly drifts to the right, move it a bit again and it stops.
I don't know what causes it but maybe someone searching for this can find a solution.
