Depends on your usecase. I’d say an ad blocker is essential, because even if you don’t install any untrusted apps, there’s still everything you do in your browser that can harm you.
A firewall can help you manage if / when / how your apps can access the net. I completely block some apps, constrict some to wifi and push others to a vpn connection.
If you are rooted and want do dive deeper into AFWall, this article 
is a good starting point.