ive only seen this a couple of times, but in any browser i have used i.e. not just firefox. it was, and still is, my assumption that it is the server at the other end limiting your ip address to 2 simultaneous connections.
get the fasterfox plugin. Then you can set it to anything you want
fwiw, to change it in IE you need to change a registry key
Also, if you play around with it a lot (and attempt to download from a lot of different, slow web servers) then you should look into increasing the number of half-open connections in windows, which is limited in SP2 to 10. Google it, and you will find a patch named EvID4226Patch223d-en.zip
From the same site?
you could be getting que'd from host.
I currently I have 5 downloads going right now, and im que'd from the same site at 2 downloads..
But im using 1.5 not sure if its a firefox 2 thing for multiple sites.