Looks like they also have some JS files in the same folder (http://mozile.mozdev.org/0.8/src/dom/InternetExplorerRange.js and http://mozile.mozdev.org/0.8/src/dom/InternetExplorerSelection.js) to give IE ranges/selections the capabilities of a W3C Range and the Mozilla Selection object, respectively, that I may have to peek at to see if I can refactor any of the RangeInformation class I put together to wrap ranges in a mostly cross-browser fashion.
Along with the Selection library I've recently created, I'd love to publish a copy of the TreeWalker to the open source world. We'll see....
Upon second inspection the next day, it turned out that parentNode, firstChild, lastChild, nextSibling, previousSibling were all actually harder methods than I had given them credit for, and nextNode and previousNode were actually the easy ones. The complications come from the whole "logical view" as defined in the W3C spec for DOM Traversal (DOM level 2) that features the TreeWalker.