If (and only if) your hosting provider has 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.
Activating this option will decrease the cache rate given the fact that the Firewall caching will respect the Accept-Encoding header. Therefore, do not activate this option if your hosting provider does not have Brotli enabled.