Firefox

It is much more difficult than that imo.

Many of the Chromium forks have small teams, sigificantly smaller and with little actual in-engine experience compared to Firefox for example.

These teams need to have sufficient resources to maintain a reasonably significant fork of a standard, which will likely get harder over time, and which none of them presently deal with, as they ride the standards implemented by Chromium so far.

Additionally they would have to maintain their own extension stores, which many presently don't.