Why am I able to run a webserver and web browser on port 80 at the same time?

After leaing about and trying sudo lsof -i and sudo lsof -iTCP, and reading "Do web browsers use different outgoing ports for different tabs?", I think I'm just reading the output from lsof incorrectly.

Here's an example:

$ sudo lsof -iTCP:80
COMMAND  PID  USER      FD  TYPE     DEVICE  SIZE/OFF  NODE NAME
...
firefox xxxx  user  xxxxxx  IPv4  xxxxxxxxx    xxxxxx   TCP 192.168.0.100:12345->stackoverflow.com:http (ESTABLISHED)

Looking at this example and other output from sudo lsof -i I think it's pretty clear that this line from Firefox appears in the output because Firefox is coecting to port 80 on a remote server, from the local port 12345. Firefox is not coecting to a remote server from the local port 80.

I don't know if it is possible for a client like Firefox to coect from port 80 while a local server is listening on port 80, but what is happening here at least doesn't seem to challenge that idea, because that is not what Firefox is doing.