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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.