Comment 4 for bug 575118

@dagurasu
I don't believe a hierarchy is what you want when using tags. I would rather orientate towards the behaviour of Zotero (a Firefox plugin for bibliography management): Make the tag list filtered such that when you select a tag (or keyword for that matter), the tag list is filtered to tags that are found on pages which already contain the selected tag. This is in fact much more natural to unstructured meta-information that tags are and even embraces your use-case. Instead of browsing a tree, you filter the taglist at each level.

Of course one is always free to implement a pseudo-hierarchy by tag name convention, although I admit, this may look rather ugly.