I think you're referencing Status -> System Logs -> Firewall Tab but the section I'm concerned about is Status -> System Logs -> System tab. Unless I'm misunderstanding what you mean. None of my firewall rules that allow or block IGMP are set to log.
Messages like this are flooding the system tab:
Dec 20 19:00:46 igmpproxy: Note: RECV V2 member report from xx.xx.xx.xx to 224.0.0.252 (ip_hl 24, data 8)
Dec 20 19:00:46 igmpproxy: Warn: unknown Mode in V3 report (10502176)
Dec 20 19:00:46 igmpproxy: Note: RECV V3 member report from xx.xx.xx.xx to 224.0.0.22 (ip_hl 24, data 16)
Dec 20 19:00:46 igmpproxy: Note: RECV V2 member report from 10.0.0.111 to 224.0.0.252 (ip_hl 24, data 8)
Dec 20 19:00:42 igmpproxy: Note: Adding MFC: 207.228.xx.xx -> 232.239.0.10, InpVIf: 0
Dec 20 19:00:42 igmpproxy: Note: New origin for route 232.239.0.10 is 207.228.xx.xx, flood -1
Dec 20 19:00:40 igmpproxy: Note: RECV Membership query from 10.0.0.1 to 224.0.0.1 (ip_hl 24, data 12)
Dec 20 19:00:40 igmpproxy: Note: RECV Membership query from 10.0.0.1 to 224.0.0.252 (ip_hl 24, data 8)
Dec 20 19:00:38 igmpproxy: Note: The IGMP message was from myself. Ignoring.