Fine, XUL had to go. But where is the replacement? How many more years should we expect Mozilla to need to implement configurable bindings? It doesn't even need to be extension-accessible, just give users a tab in the preferences menu like damn near any other application has been doing since the dawn of GUIs.
• You want to refactor XUL so it doesn't duplicate features of HTML5? Whoops; you broke all the extensions.
• You want multi-threading? What a shame; that API over there assumes it'll always be called from the main thread.
• Update that database table's schema to store more data? Bah. Make another table, or you'll break extensions
When every implementation detail is part of your interface, bad things happen.