Photographer/developer Joe Lencioni‘s interesting little project Greased Lightbox lets Firefox display clicked images in an attractive floating overlay. Once this script is installed, clicking on an image link (e.g. from Flickr or Google Images) displays the image like this. Greased Lightbox is based on Lightbox JS, and using it required first installing Greasemonkey for Firefox (or this thing for Safari, which I couldn’t make work).
A new version of Creammonkey is now out which seems to have addressed some of the issues with installation. It might be worth another shot.