how can an application use port 80/HTTP without conflicting with browsers?

Question

If I understand right, applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems. But how does that work without conflic

Answer 1

It is the 5-tuple of (IP protocol, local IP address, local port, remote IP address, remote port) that identifies a connection. Multiple browsers (or in fact a single browser loading multiple pages simultaneously) will each use destination port 80, but the local port (which is allocated by the O/S) is distinct in each case. Therefore there is no conflict.