WebExtensions is Chrome based not standards based. And Firefox had sandboxed and unsandboxed extensions before.
What Mozilla promised would have been worth it. Parity with Chrome's APIs. APIs to support popular features of old extensions. Support for extension developers to develop new APIs. But they abandoned these promises.
And they managed the transition badly. They chose an arbitrary deadline before evaluating scope or designing a migration path. They held the arbitrary deadline as they missed estimates. Extension developers didn't know when or if APIs they needed would be implemented. And no version of Firefox supported old extensions and reasonably complete WebExtensions APIs. These decisions made many problems for developers and users. And this was shortly after many developers spent much effort making their XUL extensions multi process compatible or converting them to sandboxed extensions.
What Mozilla promised would have been worth it. Parity with Chrome's APIs. APIs to support popular features of old extensions. Support for extension developers to develop new APIs. But they abandoned these promises.
And they managed the transition badly. They chose an arbitrary deadline before evaluating scope or designing a migration path. They held the arbitrary deadline as they missed estimates. Extension developers didn't know when or if APIs they needed would be implemented. And no version of Firefox supported old extensions and reasonably complete WebExtensions APIs. These decisions made many problems for developers and users. And this was shortly after many developers spent much effort making their XUL extensions multi process compatible or converting them to sandboxed extensions.