How does SOCKS configuration works on firefox?

I'm using a manual proxy configuration on Firefox to use a SSH tunnel and everything works fine. However I try to understand how does it works. From my understanding Firefox is just transferring every request to the specified port but I'm not quite sure of what does it means. Also what happen when I don't use a proxy? I guess Firefox is using an other port or something? Basically I'm trying to understand how a web browser connects to internet.