This game appears to be written in JS. I've been working on a browser game recently myself and there is definitely some bottlenecks when it comes JS execution in Firefox, especially on Linux.
Not sure why you're getting downvoted since it's completely true. Making a cross-browser game is hard.
For regular apps it hardly matters but when you need to run complicated logic 60 times a second differences between browsers and even nuances in implementations between platforms (Windows vs Mac, desktop vs mobile) start to matter. And then we have all the rendering and sound caveats.
