KHTML/WebKit/Blink has always been built with the intention of many browsers (or anything else that needs a rendering engine) integrating it, thus it’s very easy to do so.
Gecko hasn’t been built with the intention of being integrated into any browser at all. Gecko isn’t integrated into FF either. You integrate the browser into Gecko, not the other way around. It’s closer to building a browser in Electron than to building a browser with the Blink engine.
KHTML/WebKit/Blink has always been built with the intention of many browsers (or anything else that needs a rendering engine) integrating it, thus it’s very easy to do so.
Gecko hasn’t been built with the intention of being integrated into any browser at all. Gecko isn’t integrated into FF either. You integrate the browser into Gecko, not the other way around. It’s closer to building a browser in Electron than to building a browser with the Blink engine.