An uninformed plan:
- Take Tor's base Browser, the one they use as a branding-neutral base (very clever name) for the privacy-focused soft forks Tor Browser and Mullvad Browser (i know, i know - but they already paid for the work, so some good might as well come out of it)
- Rebase your soft forks on Base Browser, and benefit from that work
- Form a working group to co-manage base browser
- Hard-fork from firefox as you reject future rebases in favour of privacy and humanity.