As well as to expose a stable extension API (no more XUL extensions breaking randomly on every browser update) and to enforce any sort of security boundary/permissions system.

Note that this was done after two prior failed attempts to rewrite the CSS engine for multithread in C++: https://hacks.mozilla.org/2019/02/rewriting-a-browser-compon...

Its sad to lose DownThemAll and the plethora of othwr XUL extensions, but having performant multicore support is critical given the shift to having many cores (as single thread performance improvements become uncommon).

Which honestly makes sense. Komodo is one of my favourite editors, but Christ working with XUL is painful!