They are using a hosted provider, run by the makers of Matrix themselves. This lets them get the benefits of federation while making the hosting someone else's problem.

Of course, we all want to see Mozilla bootstrap their own Matrix instance, but maybe the cost/benefit equation worked out differently for them than we would have expected. I can imagine this lets them replace IRC with a better solution faster while still supporting the development of an open standard by becoming a client of the company that's developing it.

Does this forebode something more sinister? I hope not...