Combine plums, orange rind and pectin in a large sauce pot.
Bring to a rolling boil.
Add sugar and return to a rolling boil.
Boil hard 1 minute, stirring constantly.
Remove from heat; stir in orange-flavored liqueur.
Skim foam if necessary.
Pour hot into hot jars, leaving 1/4 inch head space.
Process 10 minutes in boiling water bath.