Honestly, this is probably the coolest extension I've ever seen for Firefox. Maybe it's because I'm obsessed with photos and this feeds that obsession, but I think this just makes it SO much easier and fun to browse through pictures online. Just tremendous.
You can use it in conjunction with almost any pictures online. Whether you're searching for images through Google (there's a search bar up top) or simply browsing through the photos on some website or someone's Flickr stream. It goes into this full-screen mode and it's fluid like something on a Mac. Absolutely seamless.
You just glide through the mosaic of photos, find one you like, and click on it. Want to see it bigger or know more? Double click on it and it blows it up full-screen. Click on the little arrow in the top left corner and it goes to the photo's URL. Again, tremendous.
When you're on Flickr (or anywhere I suppose), just look for the little blue arrow in the corner of any picture (when your mouse rolls over it) and click on it to bring up the the program in no time flat.
Like I said, this really is an beautiful extension for Firefox. It's 1mb in size, so you really should give it a go.
(Give it a shot on the Library of Congress's (3000 photo) Flickr page!)