How to take a high quality screenshot of the whole paage in firefox using GCLI?

I've been using Firefox's built-in GCLI to take screenshots of webpages, so that I don't have to use any third party tool.
It is really simple to do,

• Hit Shift+F 12
• Type screenshot filename.png , then Enter.

To take the screenshot of the whole page (including those parts that are not visible), you just have to add --fullpage to the end of the above command.
Here's where the problem is, when I take the fullpage screenshot of a large page, the quality isn't high enough that it becomes pixelated or the text is not legible when zoomed in.
Is there any way to increase the quality of the screenshots without using any third party tool?
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I'm using Windows 7 and latest update of Firefox, if that matters.

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