Firefox doesn't use one port, it's a client application so every time it initiates a connection it asks for it to the Operating System, the latter assings a different port number for every connection.
So, it doesn't matter how many Firefoxs you opened but how many connections each Firefox initiated, each connection will have a different local port.
Perhaps you get the same port every time you use Firefox because it is returning the resources to the operating system when you close it and the new execution it's getting the same port.