I don't know if I'd say separating Gecko from Firefox is all that difficult. About a decade ago I worked on a project at the tail end of my internship at Mozilla to separate Gecko from Firefox Mobile. The idea being to create a sort of GeckoView Android component that could be used like a WebView component to give devs the option to embed Gecko in their app rather than (at the time) WebKit and for Firefox Mobile to become a UI wrapper around a GeckoView component as well. I only had a few weeks to work on it and in that time I had a rough proof of concept running which was an independent Android app that ran Gecko through this new GeckoView component and had a super basic UI to control it. Unfortunately being an internship project I didn't have time to take it through to completion and being the Firefox OS days at the time the team had other priorities so I don't believe it ever got fully finished. But point being is that it's not terribly difficult to separate the two; I did it as an intern in a few weeks a decade ago.

Ah, so it isn't really built do be adapted by others the way Chromium is. Well that's too bad. At least there's Floorp although I don't really have the knowledge to actually check whether their code is fine or not (as they are quite unknown yet) so I'm not so sure whether to trust them.

Anyways, thanks for your answer! :D