It would be trivial to implement a Bookmarklet that works in Firefox (or h*ck I bet you could even implement one that would work on both!), so why didn't the MSN Spaces team do this?
While this does not represent our API strategy, we have implemented a basic HTTP URL based "API" that we use for the Blog It functionality (IE only). While this serves a different purpose than the APIs Dare is discussing, this URL can be used to pre-populate the on-line blog entry forms. The details can be found by following my blog url above (as well as an "experimental" IE toolbar button for quickly blogging on-line articles).