• set flow tcp-mss
• set flow all-tcp-mss PPPoE

For NS-5GT, SSG-5, and SSG-20 devices, the command 'set flow tcp-mss'is enabled by default to 1350. 
On all other Juniper firewall devices, the command 'set flow tcp-mss' is disabled, i.e it is not set by default in the configuration.

Enter the command 'get flow | inc mss' to see the current values.   For example, look for the following fields:

flow change tcp mss option for all packets is not set
flow change tcp mss option for vpn packets = 1350

Enter the command 'get config | inc mss' to see the configured settings.

For more information on the difference between the two MSS options, refer to KB6346 - What does set flow all-tcp-mss and set flow tcp-mss do.

Note:  If PPPoE is enabled and bound to an interface, the command set flow all-tcp-mss 1304 will be added, if there was no previous set flow all-tcp-mss command configured.