Difficult. Basically Firefox XUL allows extensions to do everything that is not forbidden, while the Chrome allows only things that have an explicit API function for it.
This is the main thing that's keeping me from using Chrome more often - apparently the extensions cannot modify the UI of the browser, which means I lose a lot of customizations. For eg., an extension like Tree Style Tab (https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...) is apparently not even possible.
Firefox extensions have exactly the same amount of power as Firefox itself.