I tried to use it ~1 month ago, it wasn't a great experience. I tried playing video and several games on Kongregate, and almost nothing worked, except Flash ads (only few games showed anything similar to loading screen -- most of them just didn't start at all). But the most problematic thing was its slowness. Even small almost static ads brought entire Firefox to its knees. I am looking forward for this project, but I really hope the developers will solve the performance problems before shipping it in stable Firefox.