It's been covered to death, but it's a vastly different approach that is closer to what other browsers provide, allowing many extensions to share the vast majority of their code across Edge, Firefox and Chrome (and descendants.) It also is highly asynchronous, which has become more important as Firefox has become multi-process and multi-threaded, and more secure, both from a usability standpoint (users have more control over add-ons) and from a low level standpoint (extensions are more 'sandboxed' than before.) There's more advantages but needless to say there's no turning back, so hopefully the missing features needed to implement what people want will arrive with time.

As for me, I'm just happy to be running Firefox as a daily driver again.