The Screengrab! extension makes your life as web developer much easier by letting you make screenshots of webpages from Firefox itself, without having to install third-party software. Simply right click anywhere on the webpage you want to save and click on the "Screengrab" menu. You can then choose between saving the captured image or copying it to the Clipboard.
Screengrab! can capture a selected region, the visible portion of the website as seen through your browser's window or, what is more interesting, the whole webpage from top to bottom -that is, even the areas that fall beyond the browser's limits. This is probably the fastest and most convenient way to capture a complete website without having to copy different areas and then paste and manually arrange them on an image editor.
The only thing I missed in Screengrab! was the possibility to save images in more than the two formats available (JPG and PNG). Also, I found quite uncomfortable the fact that the program doesn't save the file with the appropriate extension -you have to type it yourself.