If (and only) your hosting server have Brotli enabled on their server, it's recommended to enable this option to avoid issues with deprecated browsers.

You can access it under Performance -> Brotli in the Sucuri Firewall settings.

Brotli Support

Activating this option will decrease the cache rate since the Firewall caching will respect the Accept-Enconding of the visitor. Therefore, do not activate this option if your hosting server doesn't have Brotli enabled.