It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

I don't have Linux, but I am showing an update to my windows version. I wish update would be clearer for EVERY owner of the game.
I can confirm the game works both fullscreen and windowed mode on Debian 8 (Stable).