I saw the same problem occur for me on upgrade from lunar to mantic on 14 November.
In my case, snap-store remained tracking latest/stable (as opposed to latest/stable/ubuntu-23.10), but I did end up with firmware-updater successfully installed and tracking latest/stable/ubuntu-23.10.
I manually refreshed snap-store to channel=latest/stable/ubuntu-23.10, and all was well.
I saw the same problem occur for me on upgrade from lunar to mantic on 14 November.
In my case, snap-store remained tracking latest/stable (as opposed to latest/ stable/ ubuntu- 23.10), but I did end up with firmware-updater successfully installed and tracking latest/ stable/ ubuntu- 23.10.
I manually refreshed snap-store to channel= latest/ stable/ ubuntu- 23.10, and all was well.