They basically did, until the underlying browser became incompatible. That was the point: to have a stable API instead of extensions relying on implementation details, which includes multiple processes.
They didn't just broke them by necessity of rewriting the browser, though – they then actively blocked them on regular browser versions (non-Nightly/Developer Edition/etc.).
Case in point: With some limited maintenance and a compatibility shim for loading them, a number of old add-ons are still working on current Firefox versions: https://github.com/xiaoxiaoflood/firefox-scripts/tree/master...