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.