Screengrab is a useful Firefox add-on which will take a screenshot of a web page, no matter if the web page is longer than the actual screen area. Screengrab will create an image from the entire web page in either PNG or JPG format.
The latest version is Screengrab 0.96.1, which works on Firefox 3.0.x as well as on the latest stable Firefox, 3.5.x. Screengrab allows you to save screenshots either as PNG or JPG, and include the timestamp in the resulted filename. It offers the possibility to capture the complete web page, visible portion, selection or window and to use it just right-click anywhere on the web page and go to the ScreenGrab! -> Save... menu entry.
Here is how nice a screenshot of TuxArena website looks like after using Screengrab:
Install Screengrab from the Mozilla add-ons page