This extension interfaces with http://whos.amung.us.
If you use their widget on your website, the extension gets how many users are on your website and puts the count in your status bar. To configure this after it has been installed, open up the addons list from the Firefox menu, click on this addon, click options, and enter your 8-12 character key which is found in the html you were given. Ie: the image name, without the '.png' part, it's pretty self explanatory. Close options, and soon thereafter your user count will be updated in real time :)