I'm no usability expert, but to me it would make more sense to name that parameter "pdfjs.enabled" true/false

about:config isn't really optimized for usability and there's some advantage to the double negative in that the absence of a value could be treated as a false boolean value.

So if at one point pdfjs should be enabled by default, they can just get rid of the preference in the default preferences file altogether instead of having to ship one where it says enabled: true (because absence of the value would be treated as false)

Does that mean I can just remove the Firefox extension installed from https://addons.mozilla.org/en-US/firefox/addon/pdfjs/ and then turn on this setting will enable a built-in PDF viewer?