You are right : the communication goes from your computer (source port chosen "randomly") to a web server (destination port 80). And from a web server (source port 80) to your computer (destination port xxxxx) for the server's responses.
If you close port 80 in outbound rules, your computer will not be able to access any web server because this rule means that your firewall drops any packets which are send from your computer to a destination on port 80.
EDIT
Actually, the packets you send contains parameters such as :
Your_IP, Server_IP, Source_port (xxxxx), Destination port (80)
When your firewall sees that kind of packet, it applies the outbound rules (the one concerning the communication FROM your computer TO a web server). If your outbound rule is to close port 80 (which means to drop any packets whose destination port is 80) it is normal to see the packets you try to send to a web server getting dropped.
closing port 80 in outbound rules doesn't mean you close your computer's port 80. It means your firewall drops packets whose destination port is 80.