[AgentME]

>I don't think the javascript runtime for plugins should expose anything to the js running on the page, but that's a lot more complicated, since the plugin runtime is almost always really interested in spidering the page DOM and altering it by responding to the state of that DOM.

Firefox/Chrome/Safari extensions already run in an "isolated world" so that they have separate sets of global variables and DOM-wrappers, so that javascript values never leak directly and modifications to globals don't affect other worlds. However, they all see and manipulate the same content in the DOM. I'm unsure if you're proposing anything different from the current situation.