randydrobinson wrote:selecting 'all', just appeared to toggle the (4) available options.
Actually selecting "all" (which applies it to all interfaces) is not the same as selecting all four options (which applies it to all interfaces of those four types only). As 16again mentioned, you can also try the older way of using "modify" firewall (which disables offload though).
Either way, once the config is applied, check the output of "sudo iptables -t mangle -L -vn" to see the actual iptables rules generated. Then try loading a few pages and check the iptables output again to see if the counters for the MSS clamping rules increase at all, which should indicate whether those rules are actually applied to the traffic.