Schmackhaft by Michel Albert

Instead of using a hierarchical structure (folders & subfolders) to manage bookmarks, this extension stores the bookmarks in a flat structure with only tags. It offers a simple side-bar to "drill into" these tags one by one. Clicking on a tag will only show links with that tag, and also remove all tags which are not shared in common with other links.

This removes the mental burden of remembering what the "first level" of the bookmark tree was when it was saved. This also makes is a lot easier to save bookmarks because you don't need to decide how to categorise it. Just add some tags and you're done.