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.