[tn123]

I get this conclusion from over a decade of add-on development, and many years of volunteering my time to/for mozilla in various capacities incl helping other add-on developers, reviewing add-ons for AMO, fixing bugs within Firefox itself, during which I became quite familiar with the code base and development process of Firefox.

PS: As to maintaining XUL vs WebExtensions API, they always maintained XUL/XPCOM themselves because that's what Firefox itself uses, meaning the entirety of Firefox developers "maintains" that "API". They regularly broke stuff for add-on developers, which was sometimes annoying, sometimes avoidable, and other times just necessary.

Some add-on developers learned to adept to that, other add-on developers switched to the add-on SDK (which like WebExtensions is a limited API, just not chrome compatible) if it was feasible, and a lot of developers will switch to the WebExtensions API if feasible in the future or even now.