If there was a single must have Firefox extension (I currently have 8 installed), I’d have to say it’s PicLens. Especially if you spend any time on sites like Flickr, Facebook, Picasa Albums, Google and Yahoo image search. When installed, if you visit a supported site (like Flickr), any image you roll over displays a little play button (>), that when pressed scans all the images either on that page, or in the group you’ve clicked on. You are presented with a full screen display of all the images in a very elegant 3d navigation system that you can control with the mouse wheel (zoom in / out) and the arrow keys to navigate.

PicLens instantly transforms your browser into a full-screen, 3D experience for viewing images on the web.

Best of all no pagination, for example if you do a google image search for Kobe Bryant and view in Piclens, you can navigate a nearly endless wall of images without ever having to click to load a new page.
In my opinion, this is the future of the web.

Piclens even supports IE, not that anyone should be using IE. But it’s there in case someone still does.