F Zero GP Legend (rs-fzgl.gba) worked for me (FF 15.0.1).
It's running at ~80% speed, I suppose we can't expect much more from today's js engines. But given how much faster the engines have gotten in the last few years, I think this is just a matter of time.
Thanks to all developers who are making this happen!
edit: Tried one more, "2563 - Tony Hawk's Downhill Jam (U).gba". Crashed slightly later, about a second after upload.
Not sure what else to say. It's great to say JS projects like this but I can't make it work.