Webkit implementation is unmaintained and insecure, developer recommends against it. The comic is about users or developers relying on buggy or otherwise bad behavior for bad reasons - ie, extension developers mad about Firefox switching to a far more secure framework for extensions when their own extension is totally possible to implement without being able to monitor all web activity without even notifying the user, because they got used to doing it a bad way that is no longer possible. That's not really comparable to Firefox just lacking an accessibility option.

Trust me, I miss using proper extensions and not just greasemonkey scripts, I've wanted such a fork for years, but the only other reasonable alternative is Vieb which is fucking Electron based.