I have been looking for this for a while. I built a browser with Chromium Embedded Framework (CEF) a while back, but Google neutered all users of it by disallowing Google logins from it using opaque heuristics that were hard to work around.

I made a POC of using Qt to capture the FF window: https://github.com/cretz/ffembedpoc. But I fear if a browser engine becomes too easy to embed, large companies like Google will blackball it for fear of automated use, like they've done in the past.