I wish Firefox would have left the old extension abilities. They could have easily added the WebExtensions standard to allow for cross-browser compatibility and not removed the old functionality.
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.).
Chime tried to be the internal Slack equivalent for an embarrassingly long time. Slack has only been used for a handful of months, and some (frustratingly) have not yet switched over.