Ive got this feature in a browser I use already, Im not all that impressed with it to be honest as Im on a 14" laptop and it takes up real estate but some people might like it if theyve got a 19" uber screen. It gives a sidebar with a preview of each tab thats open and is rather unimaginatively called Tab Sidebar.
I personally would like to see something more along the lines of the when you hover over a tab you get a preview like you do in the Vista toolbar.
https://addons.mozilla.org/en-US/firefox/addon/6535