Browser engine diversity aside, what I'd really be interested in as a user is basically Ungoogled Chromium with a simple plugin interface for sync providers, so

Browser engine diversity aside, what I'd really be interested in as a user is basically Ungoogled Chromium with a simple plugin interface for sync providers, so you could use a Firefox account, Google account, MS account on a per browser profile basis. I want the vendor services/additions to be plugins instead of forks.