There are some (stable) APIs for developing device drivers in userspace. But you have not mentioned the main point of that document, which is that (at least according to Greg) an out-of-tree device driver in kernel space is a Bad Idea.
Is the same true for gecko embedders? Is it fair to declare that any (say) help browser ought to be developed in the mozilla tree and that the mozilla team will help maintain it? If so, then you would have a good analogy.