Krzysztof Kotowicz, a security researcher, writes about web and browser vulnerabilities such as XSS, clickjacking, CSRF, and Chrome extension attacks.
Commenters cite it for details on stripping the Referer header and for WebSocket security, treating it as a reference for web attack techniques.
It's not entirely clear to me from the post whether the reason for changing the add-on API is to get rid of XUL/XPCOM (which was inevitable) or to limit what extensions can do for security reasons and/or to make AMO reviews easier.
If it's really just about XUL/XPCOM, there are promises you (by which I mean Mozilla) could do to make developers happier:
1) That you'll figure out what will replace XUL/XPCOM in Firefox before you force add-on developers to make similar changes. Perhaps there's been some discussion of this at Mozilla since https://mail.mozilla.org/pipermail/firefox-dev/2015-July/003063.html that I'm not aware of.
2) That functionality provided by non-XPCOM-based APIs currently available to extensions will remain available, e.g. OS.File and js-ctypes.
3) That there will be some mechanism for overlays for future browser UIs, beyond simple browser buttons.
If it's about limiting what extensions can do, and not about the needs of new technology, I think a serious discussion needs to happen. There's always going to be some compromise between security and liberty, and developers would probably have understood if told that they have to write things in certain ways so that the browser itself can be more secure. But telling developers that they can't build what they want to build because Mozilla doesn't trust them to write secure code is more questionable. A bad programmer can write insecure code with any extension API. (Chrome's extension API didn't protect against…
Except an attacker can strip a referer header: if you fail open like that, you leave yourself open to attack.
See http://blog.kotowicz.net/2011/10/stripping-referrer-for-fun-and-profit.html for examples
I agree, security in Web Socket must be robust/comprehensive in both ends. This post says so too: http://blog.kotowicz.net/2011/03/html5-websockets-security-new-tool-for.html. Thanks!