I can see WebExtensions breaking it (as it's a completely new set of APIs for extensions, and the losses do definitely still hurt)... but quantum/xul? How is that related, aside from "it happened around the same time"?
IANA firefox dev: XUL/XPCOM = old APIs, WebExtensions = new (multi-browser) API
Quantum was the the project name to re-engineer Firefox internals, with lots of design changes, not just plugins. XUL/XPCOM APIs were dropped, as an occasional programmer I understand why, "Quantum broke my plugins" is a reasonable first approximation for most users.
