The gHacks blog writes up a useful tip: You can save bookmark searches in Firefox to use as smart “virtual” folders, and then pin them to your bookmarks bar.
The idea is simple: open up the Organise Bookmarks dialog through the menu or using Ctrl+Shift+B, type in the search criteria you would like to use, and then use the Save button to create a dynamic search folder. Once you’ve created a number of them, you can drag them all to your bookmarks toolbar to make automatically updated folders—so you can simply tag pages and then access them through virtual folders. For example, if you tagged an item “toread”, you could create your own reading list folder on the menu, and simply un-tag the item when you are done.
Save Firefox Bookmark Searches [gHacks]