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This isn't too far from Firefox's use of XUL.

Indeed. Main difference is the API we expose to JS. XUL use is really UI oriented. Here we want to control the entire browser through JS.

Gecko exposes a large number of its interfaces to JS, and the entire Firefox frontend is written as such: http://dxr.mozilla.org/mozilla-central/source/browser/base/c...

Do you have access to cookies, localStorage from JS?

Yes. Any XPCOM object is usable from JS that runs inside Firefox chrome and most things are XPCOM objects.