There's already insane confusion about laptop CPUs. Every time someone asks me my opinion about some laptop - I have to lookup some CPUMark score, because you can't tell anything from name, generation or GHz anymore.
My "favorite" latest (intentional) confusion is from Intel who decided to name chips "Celeron" that are both Haswell-based and Atom-based. Why intentional you may ask? Because they can begin offering Haswell-based ones at first, get high praises for the performance of budget laptops, and then in new generations switch to the Atom-based Celerons, which can be anywhere from 2x as weaker to slightly less weaker, but costs Intel half the price to make them, and they could be offering them for the same price to OEMs, since it's "Celeron" after all.
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)
