Gecko is kind of PITA to integrate on anything, unless you are just writing a glorified Firefox skin. That’s why Apple forked khtml to make webkit, despite khtml being less compatible with websites back then. It doesn’t help most Firefox forks come into almost-unusable-paranoia-driven flavour (like Librewolf) and maintainers-are-in-way-over-their-heads flavour (like Zen), and they all lack enough maintainers to keep up with upstream.

Servo could be a serious Blink/Webkit competitor, but unfortunately Mozilla dropped the ball there, and Samsung is still not taking it seriously enough.