If I were you, I honestly wouldn't worry about an additional firewall unless you're doing stuff that you shouldn't be, such as hosting torrents that are illegal, buying/selling on the dark web, etc.
A firewall wouldn't help in that case..that's what VPN's / proxies are for.
A firewall is a defense tool to keep malicious activity away from your network, which can happen as random probes, targeted attacks, etc.