Earlier quoted context omitted.
A lot of "third party browsers" like Opera, Vivaldi and Brave choose to go for Chromium as their engine, would they be able to use Firefox's engine the same way? If yes, why does it seem that no one actually does it? I personally don't particularly enjoy Firefox as a browser, but I don't have anything against their engine. If there was a version of Vivaldi that implemented that I would probably choose it over the Chr…
There are browsers using Firefox's engine; most are forks of Firefox itself, but there was Camino, Conkeror, and Galeon, plus K-Meleon which is still around: https://en.wikipedia.org/wiki/K-Meleon
(It might still be, but I haven't touched it in over a decade.)