The organizational differences between the two sites:
1) http://www.w3schools.com/xpath/
2) https://developer.mozilla.org/en/xpath
pretty clearly show why W3Schools is capturing so much beginner mindshare.
From 1), it's trivially clear where to start: the link is labeled "Start Learning XPath Now!", and even if you skip that, below there's a linear list of topics. When and if I do click on the "start" link (or the first item in the ToC), I'm greeted with what is very clearly the first part of a walkthrough. Going through the sections, it's all pretty clearly marked, chunked up into easily digestible pieces, etc. Perfect for a first glance through the topic, even though I'll definitely admit that in general W3Schools content leaves something to be desired.
Conversely, from 2), the Mozilla.org page, it's pretty difficult to figure out where I want to be clicking, assuming I know nothing. After I discard the "tools" section as probably irrelevant to learning from scratch about this, I'm left with the "Documentation" column. So far, so good.
The first item in that list says "Introduction to using XPath in JavaScript", which still sounds pretty good, except wait! At the top of the page, it says that XPath is primarily used with XSLT, but this tutorial gives an intro to XPath without XSLT, so that can't be right...why would the special case that doesn't match what people usually use this for be the first item in the documentation list?
And yikes, those other links don't really look like introductions to XPath, either, they look like special topics about XPath. The last link (http://www.xml.com/pub/a/2000/08/holman/) is probably the closest to what a beginner might want, but it's a lot of material, not the ~10 minute introduction to the topic that we'd really be looking for.
It's alright, though. If we poke around at the "Using XPath" link (https://developer.mozilla.org/en/Using_XPath), we find that "Mozilla" (yeah, I'm aware that's a wiki, but you know what I mean...) has a suggestion as to where to go to learn more:
This article does not attempt teach XPath itself. If you're unfamiliar with this technology, please refer to W3Schools XPath tutorial.
Sigh.