Think of it as mixing the web browsing of Firefox with an instant messenger.  This way you always have your close connections at your fingertips while your out on the Web.  The future roadmap for Snowl looks to deliver:

• support for additional message sources, e.g. Facebook, AIM, Google Talk, etc.;
• an interface for writing and sending messages to enable true two-way conversations;
• an API to make it easier for developers to build new experimental interfaces, e.g. an instant message view.

They have also released a couple of screenshots of the project as it looks now:

Are you excited yet, or is it still too early to tell?  Personally, it does look exciting and there is no doubt in my mind if anybody was to do it right, it would be Mozilla for sure.