There is a much simpler potential explanation for such a product management decision. Suppose Mozilla determines that 90% (made-up number) of users want addons because they want uBlock Origin. It then seems sensible to prioritize that addon and not others when determining how to spend limited engineering resources. Reasonable people can of course disagree with that decision, but there's no need to bring conspiracies into it.
(NB: Even though I worked at Mozilla I have zero insight into this particular issue; it's entirely speculation.)
