I think this entails adding an entity_picture-function to the ZaptecUpdate-entity.
It currently uses the default for UpdateEntity:

    @property
    def entity_picture(self) -> str | None:
        """Return the entity picture to use in the frontend.

        Update entities return the brand icon based on the integration
        domain by default.
        """
        return (
            f"https://brands.home-assistant.io/_/{self.platform.platform_name}/icon.png"
        )

This gives the regular light-mode icon on the firmware_update-entity which is close to invisible in dark-mode.

(Not sure what to replace it with since the backend doesn't know or care if the frontend(s) is in light or dark mode, but it seemed like a step forward to have this as an issue rather than as a # FIXME: Find how icons work for firmware-comment in the code. Maybe a separate fw-icon?)