Every connection has TWO endpoints - a source and a destination. While it's true your browser usually connects to web servers on port 80 (though a server can run on any port), that's just the outbound endpoint. The local side of the TCP/IP connection will be on a random port above 1024.

A firewall can block all outbound packets except those to port 80 and web surfing will still work. However try blocking all inbound packets except to port 80 and you'll be in trouble. "Stateful" firewalls keep track of the outbound connections and allow these inbound packets automatically. "Stateless" firewall don't so you have to leave these ranges open for inbound packets.

Also FTP, in particular requires uses inbound connections to higher ports when not in "passive" mode (making it unlikely for an ISP to block these ranges).

Actually, as long as you're not running a web server on your PC you could actually use 80 or some other common port for Gnutella - it'll work just as well and be very hard for an ISP to block.