Unfortunately, Mozilla is not interested in providing an easily embedable web engine. It makes sense: it goes against their interests as a company whose main product is a fully featured web browser.
Google, on the other hand, provides a web engine with a nice license and reasonable ergonomics that can be used for all sorts of projects. This allows them to execute an EEE strategy:
Embed (into all kinds of projects)
Enforce (moving-target standards of your own making)
Exhaust (any potential competitor/resources that needs to chase after them)
This is why I wish we can get an alternative, OSS, easy-to-embed engine soon.