Ecosystem diversity, interest in servo, refusal to let “chrome win”, privacy support baked in, etc. Why would you fork Chromium if you’re interested in Firefox?
The large number of Chromium forks vs the comparatively tiny number of Firefox forks shows that there is barely interest in forking Firefox. Especially when you look at which forks have users you'll see that it's almost exclusively Chromium forks.
Sure, ecosystem diversity and so on is great but the current forkers don't see a way to make money from that, otherwise they'd have done it.
This still doesn’t answer the question. If building Firefox is unprofitable what about Chromium based browser is profitable? Cutting out its base layer development doesn’t change its business viability. Clearly the engine development is not the problem.
Maintaining your own browser engine costs a lot of money. Using the one someone else maintains costs a lot less. It's that simple, plus the network effect, compatibility ...
Okay you've eliminated your browser engine building cost by switching Firefox to Chromium. Now how do you make profit off of Chromium?
My point is that there was no profit model around Firefox so switching engines is negligible. The cost of building said engine is also going to be negligible to the profit returned from the browser. In Google's case this is true. In Mozilla's case they have no model and never had one to subsidize that cost (other than Google search money). Changing base engines turns Firefox into a hobby project managed by a FOSS rather than a serious competing project in the browser-verse.
