Not sure about the correct solution, but the following did the trick for me.
- Close the Firefox browser if you have opened it.
- Open the terminal and run the following commands to refresh the snap.
sudo apt update && sudo apt upgrade
sudo snap refresh
This should stop the notification. As per the snap website, "Snaps update automatically, and by default, the snapd daemon checks for updates 4 times a day. Each update check is called a refresh." But it seems that you need a manual refresh to get rid of the notification.