I'd suggest that you read my colleague's comment at https://news.ycombinator.com/item?id=15696184 to understand just how much friction the legacy addon ecosystem was creating. It just wasn't sustainable.
As a user of some of those "legacy" addons, I sincerely don't care about the upstream friction. I care that Mozilla has tried to cut my user experience off at the ankles more than once. The burden of having a thriving community & ecosystem is the responsibility of stewardship over that ecosystem. Mozilla has abrogated that implicit stewardship responsibility toward their community.