I'm not exactly a Mozilla fanboy, but I do love Firefox and they seem to be on the ball most of the time and not in the camp of "constantly pissing people off". But things like BrowserQuest make me love them so much more.
From a gaming standpoint, it's not very impressive; it's a pretty standard MMO, except without levels, abilities or...well, anything that usually comprise an MMO. The impressive part, however, is that it's written entirely in HTML5. No Java, no Flash, no Flash (had to say that twice).
It's surprisingly entertaining, although not challenging, and has a ton of interwebz and pop-culture references, such as Nyan cat, the Matrix, Star Wars, and (of course) Lonely Island. My favorite little nugget is probably just the guards that have the HTML5 logo on their shield. You can tell that whoever made this game was having fun.
There are a few glitches, for sure. For example, I found that you had to play with a maximized window, otherwise you could not move anywhere. I also had one experience where my character disappeared off screen and would not come back, leaving him to die by the hands of baddies. Even with these minor glitches, the game is still very impressive.
Major kudos to Mozilla, and I think the point has been extremely well-made: you can make a great in-game browser using pure HTML5.