You can't if they have different email logins. You could use sync login on each computer to sync the data from another Firefox but if the login is gone from other Firefox then the sync will not remain. So if you want to insure your data is correct you should use only one login so they would match with each other sync. But what your doing isn't going to keep your data as soon as one is gone the sync could remove the data as well. So if you want to insure they stay you need to keep the same login but then backup the data so it can be restored back. Remember Sync isn't a backup persay. To insure your data remain is to do a actual backup should any of those computers get wiped your data will be gone from those computers.