That’s because the snap channels aren’t correctly being switched from latest/stable/ubuntu-23.04 to latest/stable/ubuntu-23.10 . The issue is being tracked at Bug #2036765 “ubuntu-release-upgrader failed to upgrade snap to ...” : Bugs : ubuntu-release-upgrader package : Ubuntu
I think early upgraders like you will need to manually change the default snaps to the correct track.
snap refresh --channel=latest/stable/ubuntu-23.10 firefox
snap refresh --channel=latest/stable/ubuntu-23.10 gnome-42-2204
snap refresh --channel=latest/stable/ubuntu-23.10 gtk-common-themes
snap refresh --channel=latest/stable/ubuntu-23.10 snap-store
snap refresh --channel=latest/stable/ubuntu-23.10 snapd-desktop-integration
If you don’t have the new Firmware Updater app, you can also install it with
snap install --channel=latest/stable/ubuntu-23.10 firmware-updater