I don't have a real answer for you, more just shades of understanding. I believe it's the case that Chromium is just more extensible and designed for reuse. Whereas Firefox is more monolithic and less able to be extended.

I don't think it has anything to do with the javascript engine per se, but more the general API surface that Chromium gives you. There might be some additional benefit that Chrome is the most used browser (currently) and so there might be some developer sentiment or familiarity with it; Chrome's popularity might push adoption for Chromium.