Is allowing port 80 on Little Snitch safe?
No.

If, for all processes.

Pretty much yes.

If, only for web browsing.



As @StuWilson has pointed out, port 80 is for http traffic and port 443 is for https traffic. (http with TLS/SSL encryption).

It's not necessary to allow all traffic of all processes on port 80. But - for the sake of comfortable browsing - I strongly recommend enabling those ports for all traffic of webbrowsers  like Safari or Chrome.

For comparison, this is my setup of Chrome in HandsOff!:


Basic communication


Allow all outgoing network connections on port 443 (https)
Allow all outgoing network connections on port 80 (http)
Allow all domain resolving

Sync


Allow all outgoing network connections to talk.google.com on port 5222

Other (e.g. media)


Allow all outgoing network connections to atracktive.collegehumor.com on port 9090 (websm)
...



Setting up a firewall can be quite a hassle. Generally - when I've expected a certain connection to build up, and know what it's for, I allow it. But you should only interfere network connections when you know what you're doing.

Many system processses regulary build up (local) network connections that are certainly valid. If you don't know how to guard a firewall, you become the first security risk for your own computer. Make sure to educate yourself on network protocols when building your own firewall.