It is not related to a specific platform, we've tested on many different computers and operating systems. We also get reports from users all the time saying the game is barely playable on Firefox but works great on any other browser they use.
Hey, I've been trying to look into this a bit. Which OS (or OSes?) do you see 10-15fps on? Because so far on Mac I'm not seeing any differences compared to Chrome, but I may just be missing it...
My main OS is OS X and this is where I've done the most testing, but we see similar performance degradation on Windows as well. We've tested on multiple Macs and Windows machines. It is almost as if no hardware acceleration is being used.