Firefox has way more than 20 developers. Looking at https://firefox-source-docs.mozilla.org/mots/index.html, if I'm not mistaken in my count, there are currently 147 module owners and peers alone. Some of those might be volunteers, but I think the large majority of them are Mozilla staff. On top of that there are probably a number of further Mozilla staff developers who aren't owners or peers, QA staff, product managers, sysadmins and other support staff…
I know they have way more than that but I'd argue that you don't need that many.
Hypothetically, if I was given the money and asked to build a team to fork Firefox I'd be more focused. Way more!
The current devs work on stuff I'd scrap like Pocket, telemetry, anything with AI, and so on. I bet there is a load of stuff in there that I'd want out! There's probably a bunch of things in Firefox Labs they're working on too.
So, I'd argue that 20 good devs (again, a number I pulled out of the air!) split into, say, 4 smaller teams could achieve a shit load of work under the right circumstances, with the right leadership and so on.
I'm currently a senior architect with over 50 devs below me. Most are mid-level at best (not a slur, just where they are in their career!) but the few good ones are very good. A team of 20 of those could pull it off!
It'd be a tall order building a browser from scratch with 20 devs maybe but it's already built.
There's someone else right now who is going to important organizations they obviously don't understand, making wild claims about 'I could do it for much less', and cutting personnel drastically.
You severely underestimate the engineering cost of modern web browser. Assuming a sufficient value-addition fork, a team of 20 cannot even catch up the Chromium upstream. Good luck coming up with a new engine compatible with Chrome; MS tried it and finally gave up.
