Taking a whole webpage into a picture is something we might need once in a while and FireShot takes good care of this. But somehow the developer doesn't support Firefox in Linux which render this add on useless even though it's installable. Luckily we have another alternative - Awesome Screenshot.
Awesome Screenshot works out of the box in Firefox for Linux. Installing is a breeze and the 'Edit' option will be available everytime you took a screenshot which enable you to crop the image and selecting only the portion you want to have.
Screenshot above displays what are available in the options of Awesome Screenshot.