It doesn't look like anyone is really using nested event queues.

Hrm, Darin spent quite a bit of time re-implementing them after he removed them the first time because we needed them for some stuff (modal dialogs, or sync XHR, or something?) Please tread carefully!

What are we actually talking about removing here? nsChainedEventQueue? Something else?

this bug is to investigate and possibly remove: http://mxr.mozilla.org/mozilla-central/source/xpcom/threads/nsIThreadInternal.idl#60 And related bits.

That code is essential for synchronous XPCOM thread proxies to work correctly. While I would love to remove those, that's definitely a post-FF4 task.