Today I watched a short video about how Mozilla is moving from IRC to Matrix by Rubén Martín from Mozilla Reps. The slides are available as well.
As far as I know, this is how the change unfolded:
Now the Matrix page in the Mozilla wiki explains how to connect and the fact that federation is enabled.
I found the video and slides above in the #general channel. It seems to be possible to link to a specific message (16:13:11 on 21 Feb 2020 if I have trouble finding it again).
The meta channel to talk about Matrix at Mozilla is called #synchronicity.
Here are some channels that seem to have public logs: