Tell users a snap has been updated?

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.

Totally agree!
When I see "Updating snaps" without any indication about which Snap is being updated I feel like they are hiding me something I don't like it.

I think that 'updating snaps' just means checking for any updates, not necessarily that any are in fact updated.

Argh, there's been another update to Firefox that I wouldn't have known about if I hadn't opened a terminal and done 'snap changes'.

Something, somewhere obvious, should tell us.

My recommendation would be to make any suggestions in regards to how snaps work at Ubuntu Discourse and/or Snapcraft Forum.

Changes to how snap functions are beyond the purview of the Ubuntu MATE Team.