You may use multifield-classifier to classify and prioritize traffic based on port numbers. Below URL explains the same.
http://www.juniper.net/documentation/en_US/junos15.1/topics/example/cos-firewall-filter-multifield-classifier-security-configuring.html
http://www.juniper.net/documentation/en_US/junos15.1/topics/example/cos-firewall-filter-multifield-classifier-security-configuring.html