Allow me to deconstruct Turgy's joke, which will make it even funnier, because that's how jokes work...
Mooseman found that he is able to use a deprecated web browser to circumvent a firewall. This presumably works because the firewall, being newer software, isn't compatible with the old browser. Turgy, noticing this, extended the same concept to an absurd extreme by expressing that the same would hold true for an extremely deprecated browser. The humor is that although the logic is sound, the idea is ridiculous for other, unstated reasons. Mooseman would not likely be running a machine that has Netscape Navigator installed. And if he did, the firewall probably wouldn't work with the browser, but the browser itself would probably be unable to display the current iteration of the CPA website properly anyway. So, in a silly and roundabout way, Turgy has poked fun at Mooseman's use of obsolete software, and done so with only four words. Such is his genius.