The game has mouse acceleration hard-coded on. There doesn't seem to be any way to disable it.
I tried every trick I could think of. Like switching from windowed to fullscreen and vice versa. I tried different polling rates on my mouse (125, 250, 500, and 1000). I messed with the values for MouseSensitivity, MouseThreshold1, and MouseThreshold2 in user.reg. None of that made any difference.
A lot of googling took me to exactly one other person who had the same problem. It was on the official forums and the others there said that person was the first to experience it. Now it seems I'm the second.
proton 3.7x1 2018-08
Freedom Force and Freedom Force vs. the Third Reich display a black screen when started for the first time (opening a single issue as they seem to use the same engine under the hood).
Starting the games with launch option
-w(windowed mode), selecting the native resolution, starting again with-wwith that resolution and then starting the game again without the-wlaunch option allows the game to run full screen without issue.System information from steam