With Thumbnail Expander, you no longer have to constantly use the back button when viewing a thumbnailed gallery. Instead, simply hover your mouse over a thumbnail to see the full image, and move your mouse away to have the full image disappear. Check out the other preview images for an example of this.
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.
Note that many, but not all, websites work with Thumbnail Expander. If your favorite site doesn't work, right click on the magnifying glass icon in the bottom-right of Firefox, and "Suggest this page".