Ordinarily you don't have to manually back up your bookmarks. It's done automatically. If people lose their bookmarks, I suspect they don't know where to look for them. The folder where they reside is not located under Program Files.
In Windows 2000/XP you can check to see if yours are being backed up by looking here: C:\Documents and Settings\[your user name]\Application Data\Mozilla\Firefox\Profiles\[random characters].default\bookmarkbackups
Mine are backed up daily although only 5 or so backups are stored at any one time.
To use a backup file, remove the "date" portion of the file name and insert it one folder up.