Comment by m4rtink
Afaik the problem is the lack of embedding API in the Firefox core, so they have to adapt their patches every time they need to update Firefox version.
This is also why you see Chome being used as a core in all kinds of applications and frameworks - AFAIK it has the necessary API for this.
There's GeckoView which one'd use for embedding Firefox in an Android app. Can't use that on Sailfish OS of course, but it'd help in figuring out what of core Firefox to bind to to make a similar layer.