The 40gb total are both already reserved, and a normal user isn't supposed to modify it so it shouldn't fill up.

For desktop apps, Vanilla will primarily stick to Flatpaks, so Firefox will also be a Flatpak.

VanillaOS already has a custom boot menu that can be used to switch slots in case an update went wrong, so that you can go back to your older, but working system.

The partitions are also not synced.

If you install something using abroot (e.g an update) it will only be installed to the unused slot. So if you run abroot --update or use the included updater, and you're in Slot A, it'll modify Slot B, and vice versa.