Is there something in Firefox's license that prevents forking? Why does it seems like all of these browser are always Chromium forks and we never see Firefox forks go mainstream?
The origins of Chromium come from KHTML which was designed to be embeddable and quite modular. Both Webkit and Blink forks retained that feature and improved it.
Chromium has V8 which is the faster JS engine (so it accommodates crappy bloated webpages better). It achieved HW acceleration early on, it is quite important for video decode. Many webdevs only optimize for Chrome for viewing.
Also: Tor Browser, Konform Browser, Mullvad Browser