Currently, it's not possible to delete the description of a firewall description:

On a testing VyOS:

configure
delete firewall
set firewall name test description test
commit

I expect the description to be deleted when I run this Ansible task:

- name: Delete description
  vyos.vyos.vyos_firewall_global:
    config:
      group:
        address_group:
        - name: test
      state: replaced

But nothing happens.

I already have part of the code ready, which I'll upload to GitHub and create a PR later today.