...still waiting for the ephemeral "Link To This Post" button to reappear...

I'm still getting just a dark grey square (Firefox 33.0.2, all plugins and addons disabled)

This might have something to do with Flash. Copying to a clipboard is actually extremely complicated and most libraries that handle this use some sort of flash/javascript mixins. I'd make sure flash is installed through your browser. Note: The flash plugin must be installed on the browser in use. 
http://get.adobe.com/flashplayer/

On a side note: AdBlocker will block .swf files if set to do so and sadly a .swf actually replaces the button behind the scenes. You can see this if you right click the button, it will actually give you an Adobe Flash Player menu instead of basic browser menu like the buttons beside it.