Firefox 3 lets you tag your bookmarks, but it doesn't give you a great way to browse your bookmarks by their tags. TagSifter tries to. Select a group of tags in the sidebar or menu, and TagSifter shows you all the related tags and bookmarks. Or search your bookmarks with set operators, like "tag1 - (tag2 + !tag3)". Comes with a tool to help tag your existing bookmarks as you move to Firefox 3.
This add-on is not compatible with your version of Firefox because of the following: