Firefox Panorama has gone through alot of name changes, last one was Tab Candy.  This add-on, which will come built in with Firefox 4 when its released, allows you to group your tabs together based on whatever folder name you want.  Kinda like your virtual desktops for windows but you can even do more with it.  Here is a good link with some info on it and a video.

Tab Candy at Lifehacker

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