Before you come at me with stuff like Librewolf, Waterfox and IceCat; those don’t count. They are just tweaked Firefox distros with mostly basic low level changes. Not every Chromium browser is super unique either, but I feel like there are more differences between them then there are with Firefox distros. Why is that? Why there aren’t different browsers that use Firefox’s engines but provide a different UX?
It would be nice if Mozilla could make a project similar to Chromium Embedded Framework, in order to integrate the Gecko engine in apps (because currently Blink and V8 are used and the truth is that the performance is not very good) and even in game consoles Gecko could get a good market share. Servo was an step in the right direction.
For Android there is https://mozilla.github.io/geckoview/. I don’t know if there is anything similar on desktop.