The Smart Productivity weblog posts a simple Firefox shortcut: You can quickly copy any group of bookmarks to your clipboard—from a bookmark folder to every single bookmark you've made—by right-clicking your folder of choice and selecting Copy. Want to copy every bookmark to you clipboard for a quick backup? Do the same thing on the Organize Bookmarks entry. When you paste your bookmarks, each URL is organized by folder name.
This worked without a hitch on Windows, but in OS X you can't right-click the menu item, so you'll have to stick with the bookmark folder in the bookmarks toolbar or the Organize Bookmarks sidebar. This may not be useful every day, but if you've done some quick research and bookmarked pages in a folder, this is a quick way to email those links to a friend or to yourself for access from a different computer.