Sun did have a firewall by the early 90's. It had application-level proxies, and you'd have to configure applications to bounce through it if you wanted to get to the Internet. In many ways, this was more secure than today's default for firewalls where you can make any outbound connection you want but only the inbound connections are filtered.

Note that I'm not arguing that Sun was a leader in security, but they did make some efforts that other companies didn't.