By Default the latest version of Firefox makes a backup of your bookmarks. The problem is that this backup is in that hard-to-find area on the hard drive under C:\documents and settings\UserName\Application Data\etc. etc. etc.(Technical data here: http://www.mozilla.org/support/firefox/profile#locate)
So instead of trying to figure out how to find that file and back it up try out the Bookmark Backup extension located here: http://www.pikey.me.uk/mozilla/
After installing go to Tools->Add ons (Or Extensions is your are running an old version of Firefox.) and tell it where to backup your bookmarks to. I chose a network server folder that I know will be backed up nightly.