[edit firewall family ethernet-switching filter block_mDNS]
set term blocknDNS from destination-address 224.0.0.251
set term blocknDNS from port 5353
set term blocknDNS the discard
set term accept_other then accept
[edit vlans <vlan_name>]
set forwarding-options filter input block_mDNS
You may also want to create one for IPv6. you can use the port so it looks at both destination and source port or use destination-port
FF02::FB
Port 5353