Well, there are also the mobile variants of Firefox, which are more of their own thing.
IMO Mozilla limited itself a bit too much on Firefox. Which results it their web engine not attracting many developers for it outside Mozilla.
Embedding gecko in your own app was much easier in the past. This is now mostly taken over by CEF and WPE for Blink and WebKit respectively.
Also stuff like B2G (Boot 2 Gecko) or FirefoxOS are dead as well.
A goal of open source should be to be hacker friendly as well, were currently Blink/WebKit is leading. There are so many more projects around those engines than Gecko, which is sad.