Old extensions won't ever really work again, but part of me is still kind of holding out hope for WebExtensions API features for things like "allow extension on 'system' pages (new tab, settings, etc)", "focus addressbar", "hide/replace address/tab bars", etc.

Some of those are coming or pretty much ready (tab hiding, addressbar focus), and others I've almost given up on, given how Mozilla seems to have changed their philosophy.

We're waiting for the webextensions API to be extended with a bunch of missing functionality so that extension authors can reimplement things that were made impossible when the older APIs were deprecated. This is something that ideally should have been done before or shortly after the quantum release but was not seen as a priority.