>>6
I don't believe they use Discord because they think it's actually good. My theory is that they're actively trying to distance themselves from the crusty dinosaur image of IRC and get into whatever glossy jank kids are using these days in hopes of attracting young blood to recruit. IRC is often cited as a barrier to entry for first-time contributors, just like mailing lists.
Mozilla itself was also considering moving to Discord after they decided Zulip and Mattermost aren't good enough. It appears there was enough pushback that they switched to Matrix instead. It's a pity they had to shut down MozNet.