Userscripts don't have access to your disk. There are browser extensions that do what you describe, use them.
In Firefox : no need for addons
Press Shift+F2
In the command line type :
screenshot --fullpage
The captured image file will be saved in your download folder
...
Press Shift+F2
In the command line type :
screenshot --fullpage
...
TIL. thnx
There's even a button on the right of DevTools (F12) toolbar if you enable it in devtools options.
Need a script that screenshots the whole webpage.
Need a script that screenshots the whole webpage from a button which clearly says screenshot or from the context menu of webpage which clearly says screenshot. In both cases it should let me choose the location of where to save the file to. And it should let me screenshot the webpage as jpg with %100 quality or png with %100 quality. If you make a screenshot script, please let us know about the quality of the saved image.