Thumbnail Expander is a Firefox extension that displays the large version of images when you hover the mouse over thumbnails. No need to click on thumbnails to display the enlarged version of images. Instead, simply hover your mouse over a thumbnail to see the full image, and move your mouse away to have the full image disappear. Thumbnail Expander also works with video links. Click on the link and have the video show up in the current page, and click the hide link to have it disappear. View the demo on their site.
The bad part: Thumbnail Expander doesn't work on all websites. It doesn't work with thumbnails that doesn't directly link to the image. For example, it doesn't work with Flickr. It also doesn't work with blogger blogs. However, it works with Wordpress blogs and sites, like Google Image Search, Wikipedia, Facebook, Imagehack and Photobucket and a few others. It also works with video links like YouTube and Google Video.
Thumbnail Expander can actually save you lots of time if you frequently use Google image search or browse thumbnail gallery sites.