I'm curious, what's wrong with IRC? Is it too old? Too difficult to monitor and moderate? Too easy to spam? I don't understand. I've never really understood why mozilla chose their own servers and not freenode. I've never really grasped the whole history of enet vs freenode either.
Having to run your own IRC bouncer increases the barrier of entry, as does nickserv based auth. Modern people also expect to be able to seamlessly post images, large code examples, files, etc. Some people also want support for threaded conversations. None of this is offered by the typical IRC setup which Mozilla probably thinks that it scares away users. I do not think Matrix solves all of these yet, but it solves some of them.