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.
➊ Click a bunch of tags in the sidebar or menu to see the bookmarks and other tags that are related. Use the related tags to quickly filter your search.
➋ Or, if you can handle real ultimate power, search your bookmarks by combining your tags in arbitrary and interesting ways with set operators. For example, to see all bookmarks you've tagged as being about muffins but not about cookies or brownies and whose names contain the word "donuts":
muffins - (cookies + brownies) ?donuts
TagSifter autocompletes tags as you type them, iTunes-style.
➌ TagSifter also comes with a tool to help you tag your old bookmarks as you migrate to Firefox 3. It works by tagging bookmarks based on the folders that contain them. Like, if your Ramen Noodles bookmark is contained in folders Eats/Cooking/Chinese, the bookmark will be tagged Eats, Cooking, and Chinese. You can tell TagSifter the root folder at which to begin tagging.
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TagSifter comes with a sidebar and a menu. Each has a toolbar button; add the buttons to your toolbar from View > Toolbars > Customize...
The tagging tool is accessed from the Tools menu and TagSifter's options window.
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For help, known bugs, and much more info see TagSifter's homepage.