Sorry to be nit-picking, but it was wrote by two Interns at Opera, not Mozilla.

The dev preview is really rough, there were no enemies, just the level to run around in.
Simple games which currently require plugins like flash or the unity player will sooner or later be using html5 or similar, built in technology. Thats my personal prediction.
Maybe Unity comes around with a way to publish to html5/webGL in the near future Well, to be honest I don't think they will. Too much overhead when you write an interpreter for a script language in another script language. They would need to "compile" the unity script to ecmascript or whatever scripting html5 uses.

But larger more demanding games will probably continue to be stand-alone, even though online distribution might make classical installers obsolete soon.