It seems like the closest thing to this idea currently is Firefox CSS. [link] Honestly, if they would just officially add support for it (show it in the store), I would just use that.
I can't really see having a whole other Firefox browser, but I think it would be nice to have some of the features shown in Opera GX like the limiter on how much RAM and CPU usage the browser can use. And of course, performance improvements are alway...
