Well, it is defined by Mozilla Labs as, “An Experiment with Messaging in the Browser”.  They launched this new side project yesterday, and I do have to say – like most things they do – Snowl looks very interesting.
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 [...]