In that case it sounds like the real explanation for forking was "the relevant part of the Firefox codebase had too much tech debt to make contributions so we had to fork our bodges." (No offense wrt tech debt; I know what it's like to be saddled with it and congrats on addressing yours.)

I hope to soon see a cliqz blog post about their project unforking, only spun as an ingenious way of saving memory, improving security and extending reach.