The new Firefox add-on Badges on Favicon takes advantage of that idea very well.  It allows you to put a rounded badge over the tab favicon of some pages.  Where does this come in handy?  Well what if you could see in your tab the number of unread Gmail messages you had?  Or how about if it told you how many new posts were in Google Reader?

Nuggets of Information for Your Tabs

The extension uses little scripts, called badgets. The badgets define both the appearance of the badge (color, position, size of the text, etc.) and how to get the data from the current page. The extension comes with three sample badgets that work with Gmail, iGoogle Reader Gadget and Zenbe. But you can install your own badgets and even create your own for your preferred site.

This add-on was also a runner up in the Extend Firefox 3 contest.  It is still listed as “experimental” so you might need to visit BugMeNot for a user name and password (or sign up on Mozilla.org) to use before trying to download.