Answer:
The message "Unhandled link-local or multicast IPv6 packet dropped" means that either link-local or multicast IPv6 packets are being dropped by the SonicWall firewall. Most of the time, these types of packets are used to talk between devices on the same network segment. They are not meant to be sent to other parts of the network.
Even though firewalls often drop these kinds of packets, it is possible that the firmware update changed the way the firewall works, which is why more of these messages are being logged.
If you don't have any problems with your network's connection or performance and don't see any signs of bad behavior on your network, it's likely that you can safely ignore these messages. But if you are worried about the number of messages being sent or if you are having other problems, you might want to contact SonicWall support for more help.