Using cron to backup Thunderbird emails

My bash script (which I've saved as backup_emails.sh in my home folder and made executable) looks a little like this:

 set -x   
  x=`date +%y%m%d.%H%M`   
  tar zcf thunderb-mail-${x}.tgz ~/.mozilla-thunderbird   
  mv thunderb-mail-${x}.tgz /media/MY_HARDDRIVE/email_backups/   

Then invoke in cron using:

 crontab -e   

add something like:

 30 4 * * * ~/./backup_emails.sh  

which does the backup at 04.30 every day