Box.net Firefox Bookmark Synchronizer
Box.net just made a new extension that backs up and synchronizes your Firefox bookmarks.
https://addons.mozilla.org/en-US/firefox/addon/5525
Its very simple, you can set it to auto update your bookmarks so they are synchronized wherever you go, or you can manually backup and restore bookmarks. The bookmarks are then stored on Box.net and can be accessed from anywhere for easy downloading.