I just set up a Mint VM and tried it in FF / Mint and it works for me. Can you give me any more information about anything that might be weird about your setup? Browser extensions, weird keyboard, anything like that?
That is a solution to your problem, but comes with problems of its own: it would cause the game to eat keystrokes it can't handle and shouldn't, like pressing F12 to bring up a console, or ctrl-d to bookmark, or whatever. Since the problem only seems to happen if you have that setting set (which doesn't seem to be the default since I've never heard of it), I suggest clearing that setting, rather than me breaking non-game keyboard input for everyone else.