The packets with the "all Fs" as addresses are broadcast packets that are sent out periodically by AppleTalk bridges. The Routing Table Maintenance Protocol (RTMP) is used by AppleTalk bridges to maintain their routing information to enable them to send packets to the appropriate bridges and networks connected to those bridges.
It is part of the RTMP specification that, every 10 seconds, each AppleTalk bridge sends a broadcast RTMP packet in order to keep the routing information for the network current.