> Ironically Firefox killed their extension API to be compatible with Chrome's extensions.

This is misleading. They deprecated the old API primarily to be able to do multi-process and get rid of XUL.

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!

I recently used the webRequest API in Firefox and it is already more robust than the Chrome equivalent. For example, you can write stream filters to process request responses. No so in Chrome, where you have no such capability.