Is it still completely necessary to run a firewall if i am behind a router. If not what kinds of attacks would hackers use against my computer. I have no ports forwarded. This is what i get when i nmap it:

Interesting ports on (192.168.1.1):
(The 1600 ports scanned but not shown below are in state: closed)
Port State Service
80/tcp open http
Remote operating system guess: Linksys BEFW11S4 802.11B WAP
Nmap run completed -- 1 IP address (1 host up) scanned in 10 seconds. I also nmaped my own ip adress (12.216.etc) and got the same result.