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?

Not license, it is the codebase and tooling. Waterfox and LibreWolf exist. However FF code base is a bit a mess compared to Chromium. Chromium has a better build system using Starlark. Starlark is quite resilient against exploding complexity and easier to keep C++ codebases sane. Mozilla's build system is a soup of multiple tools. Firefox used to use Mercury and only lately switched to Git.

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.

There are Firefox forks out there. LibreWolf, Pale Moon, and WaterFox are the bet known. Unsure which, if any, are still active.