People used to build a lot of software around Gecko, there are still some notable users like Komodo IDE, but Firefox is a lot harder to embed than it once was. Servo from the Rust team was supposed to solve this by providing a new embeddable browser core, not sure if that is still the long term plan

Firefox is the only thing Mozilla has ever been able to make any money with; anything else has gotten them a pittance at best.

Giving up on that because it's 'too hard', without first proving they have an alternative? That would be insanely foolish. They may as well close up shop now if that's their plan.