Funny looking symbol

I've noticed that sometimes when I view a thread on AAM a funny symbol appears in the address bar infront of the weblink. It appears as a box with a green background with the square root symbol in it (I cant post attachments, so apologies). Does anybody know what it is?

Website is " Powered by: vBulletin Version 3.0.7" so maybe it's their logo. I get logos in my address bars since I changed to Firefox . . .

This is the Website Icon (stored in a file called favicon.ico in the website root directory). This is functionality that has been around for a while but not worked right in Internet Explorer. If you save a link with a icon in Firefox, the icon is displayed along side the link in the bookmarks.

Thanks folks.