The new extension framework for Firefox was necessary to get rid of legacy codebase and its limitations are there to prevent malicious addon behavior and other security issues.
All of the undinspensable addons (uBlock Origin, uMatrix, HTTPS Everywhere, NoScript, Privacy Badger...) have survived the framework change. Very few interesting addons have stayed legacy-only.
Change isn't always bad. Sometimes you need to start from scratch.
@Siphonay
Now I just have to wait for SeaMonkey to catch up.
Regarding change in software: I tend to be sceptical because far too many times I've seen developers embrace change for the sake of change with no regard for how said change affects the users. Examples: systemd, Orbot.
I say if you're going to make major changes then
1. Think about how it will affect users
2. Come up with a plan and stick to it before & while writing code
3. Avoid feature creep by limiting the scope of change
