I see this as well. Follow the steps below to fix this for Ubuntu 23.10.
Note: The same commands also work for Ubuntu 24.04 (and most likely future versions). Just replace any reference of ubuntu-23.10 with ubuntu-24.04 (or the version you are using).
Run the following commands:
snap-store --quit
sudo snap refresh snap-store --channel=latest/stable/ubuntu-23.10
If despite the snap-store --quit this tells you that it cannot do so because it has running apps, something along the lines of:
error: cannot refresh "snap-store": snap "snap-store" has running apps (ubuntu-software), pids:
3071
then run:
kill -9 3071
or whatever PID is being shown, then once more:
sudo snap refresh snap-store --channel=latest/stable/ubuntu-23.10
That brought the App Center in for me.
On to firmware-updater. On my 23.04 → 23.10 it failed during do-release-upgrade. This was easily remedied by just running that install once more once 23.10/24.04 was installed and had rebooted:
sudo snap install firmware-updater
Lastly, if you're keen, you can:
snap list --all
and then run:
sudo snap remove SNAP --revision=REVISION
for each disabled snap.