I am a Firefox developer, and you're spot on. Previously there were separate hg repos for central, beta, release. I think ESRs too. And autoland. Now they're all branches in the same repo, and central is renamed main.
Commits land in autoland and get backed out if they cause test failures. That's merged to main ~twice per day when CI is happy
I've mostly encountered these branches/repos when checking commits linked to Bugzilla tickets, and I don't recall seeing "autoland" show up too much in those cases.
The "new" git default branch name is 'main' and 'autoland' existed before next to 'mozilla-central' and is the one where commits usually appear first.