How? Mozilla has kept the old webRequest API: https://blog.mozilla.org/addons/2021/05/27/manifest-v3-updat...
Mozilla got rid of XUL because it wouldn't work with the multi-process model of Chrome they were copying in order to speed up the browser for running complex javascript applications. The security justifications were nonsense. The real security problems are in supporting all the new attack surfaces that modern browsers do in the form of exposing bare metal (or just above) functionality for acting as an OS (webgl, websockets, etc) instead of a browser.