What value does this really provide? The times that I've gotten into browser extensions were really working with features that are unique to the given browser.
Extension support and depth of extension library have traditionally been the big pain point with switching to a minority browser. Moving extension support into a browser independent standard removes a lot of the selective pressure toward a monoculture.
For example, Chrome doesn't support sidebars. Opera does. So, Opera constructed a sidebar API. Then came along Mozilla, also wanted to have sidebar, and so they adopted Opera's sidebar API, allowing extension authors to easily support Firefox and Opera.
They can't support Chrome, but it's not like people didn't previously already write sidebar extensions when they could only support one browser at a time.