PicLens, by cooliris is one of my favorite Firefox add-ons. Searching images on the web is such a better experience through PicLens. Google image search is amazing with it (as is Flickr)...no longer page after page of results, but rather one continuous wall of images that can be zoomed into and out of. The slight reflection below the wall makes it appear even more three dimensional.
Not long after posting, I decided to try out Firefox 3. Then as I launched Firefox it notified me that there was a PicLens update. Apparently it's up to 1.7 now. The main difference I noticed is the "Discover" functionality, which gives you a small variety of "channels" to browse visually across the web, including but not limited to: US News, International news, movies/TV and science/technology.
By the way the install of Firefox 3 was seamless, and the default theme is pretty slick...