Bookmark Tags does pretty much what you'd guess: It lets you organize your bookmarks with tags instead of folders. When you create a new bookmark you can specify as many tags as you want, and then when you want to find a bookmark you can either select the tags you want or type them in. There are also quite a few more advanced features, so even if you're happy with your current system (be it Firefox's built-in bookmarks or a site like del.icio.us), it's still worth checking out.