This is just false.
Firefox's old add-on system gave add-ons pretty much direct access to Firefox internals. Changing just about anything had high potential to break add-ons because there was always some add-on that relied on a particular implementation detail. And Mozilla did spend effort to avoid changes which would break add-ons.
In the end Mozilla adopted WebExtensions, providing a stable API for add-on developers to develop with. WebExtensions are much less powerful than traditional extensions, but don't tie Mozilla's hands. This was soon followed by "Firefox Quantum", a bunch of performance improvements Mozilla can finally make when they can change Firefox internals freely without breaking extensions. Quantum is much faster than old Firefox versions, showing how much old add-ons were holding them back. (However, I agree that the power of old extensions was very nice, allowing many fantastic add-ons such as Tab Mix Plus. Losing them is still a bitter pill to swallow.)