This article provides information about the support extended by Juniper firewalls for MPLS.
As of ScreenOS 6.3.0 , 6.2.0r3 onwards:
Firewalls that are running in the Route (Layer 3) mode do not support or cannot pass MPLS packets.
Firewalls that are running in the Transparent (Layer 2) mode can pass MPLS packets, when the VLAN1 bypass-non-ip option is enabled (set interface vlan1 bypass-non-ip).