Using Abduction!

Download here.

Abduction! is a very useful screenshot tool within Firefox to convert a part of a webpage into an image.
It has the advantage that, unlike print screen, you can capture a whole web page, including where you would previously have to scroll and take another print screen and manipulate them together.

Capturing a screenshot using Abduction!

Right-click on the page you want to capture and select "Save Page as Image"

Then select the area you want to capture. It can be small:

or large:

You can easily change options for image quality and selection area at the bottom left hand corner:

Then simply save where you want the image to go:

And you're done!