Missing Pieces) opens up a nice prospect, of being able to script client-side with C#, and perhaps, Boo, and other CLR languages.
I think Mono fits the bill nicely on how to integrate with Firefox in Linux and elsewhere it is available, the same way it will be possible to do with the CLR in Windows Vista.
In truth, Mono's embedding API is a proven solution that, AFAIK, predates any MS disclosure of such a thing for .NET.
We just need to understand that this IE7-born integration, that is being "opened" to Firefox, isn't just some basic embedding API, but surely comes with a HUGE Object-Model attached of what is available in the browser to be scripted, and also some Security-Model which the central piece may be a list of running-time limitations and general assumptions.
Lots of fun, I foresee