It should have been obvious to me that, because of the sandboxing, having a new Firefox snap arrive wouldn't stop a running one from working properly in the way that installing a new .deb version often did/does stop a running .deb version from loading anything new.
But this means that there is no indication that there is a new snap version available: whatever does the 'snap refresh' behind the scenes, Software Updater or a cron job or... (update: ah, it's the snapd daemon), does not tell the user that one has been updated. I have Software Updater set to tell me before doing updates, but while it tells me about new .debs, it doesn't say a word about snaps.
So using 'snap changes', I discover that the FIrefox version 100 snap arrived at about 17:30 BST yesterday. Had I not done that, I would have carried on using the version 99 snap until I needed to do a reboot for a kernel update or similar, because I don't close Firefox unless I have to.