Yep, a federated social network is indeed an ambitious problem, perhaps Mozilla would've been well-suited to tackle it. The problem is not the tech or scope, but timing. 15 years ago everyone was happy to be on FB / Twitter. 10 years ago, Microsoft just bought LinkedIn; Google tried, then killed off a network with 500k DAU; all of that time, there was little space for a new contender.
Mastodon only took off because Twitter went to shit real fast; still most people flocked to mastodon.social, because they heard Mastodon was good, but had no idea what federation is, or why it's important. MAYBE that would've been the perfect timing for Mozilla to launch their own ActivityPub platform.
Yahoo had a major collection of properties that still had relevance, and core services like email, search and maps that I remember Matt Yglesias (of all people) insisting would have been the keys to the success of a FirefoxOS. Yahoo had the infrastructure, but no vision and a bad brand, and Mozilla was the inverse. An interesting what-if that unfortunately amounted to nothing.
Maybe the ideal technical solution would not require them to know?
An open slack-alike also seems like a good fit for them.
Alas, they have tons of cash but little capacity to do anything useful.