The code is already prepared very well to be embedded into something. I remember trying to embed the javascript engine SpiderMonkey into a project (I needed C bindings which I then could use in Erlang). After a week or so trying and extending, etc. we gave up and tried V8 which we had running within one hour with good documentation great APIs and so on.
I myself have been Firefox user since Firefox came out but trying to embed it myself and failing I kind of get why others choose Chromium/Blink as their base.