Here is a small free Firefox extension easy to use and very convenient, which allows you to save with one click a party (the visible part or a just a selection) or your entire web page : Screengrab.   With this plug-in, it is the end of "Print Screen" !  

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Much better addon... FireShot

https://addons.mozilla.org/en-US/firefox/addon/5648

+ more formats
+ very intuitive editor
+ it can screen visible or entire area
+ controls
+ free version is 99% similar to paid

- maybe to complexive
- it has "premium" paid version