They really can't, though. There's a pile of corpses of people who tried to embed Gecko, because it turns out Netscape/Mozilla is really bad at keeping a stable ABI. I doubt anybody remembers K-meleon anymore, for example. As far as I know Servo was supposed to be the new, embeddable engine, but that's mostly cancelled now and the Rust bits are just being rolled into Gecko (via Project Quantum).

I believe that's why Webkit was based on KHTML… the Mozilla developers Apple hired realized that had a better chance of working.

Maybe Mozilla has changed since all that happened and they're more friendly to embedders now? I've been out of the loop for a few years at this point. I don't think I've heard anything though, and they've built up a reputation…