I was playing on the browser window, the game would not go to full screen, the problem is related to mouse capture, if the game window was focused and the mouse captured, then mouse sensitivity would skyrocket to ludicrous speed :), if the window would lose focus (by pressing escape) then the mouse movement would be somewhat fixed. I also have a multi-screen setup, I don't know if that could contribute to the problem. I'm using Firefox on Linux and KDE Plasma as DE.