Number of connections can be limited in firefox. Enter about:config into address line and then edit value for following lines:
network.http.max-connections-per-server
network.http.max-persistent-connections-per-server
Lowering these values could help (set the first to 4 and second to 2 for testing and never set lower value than 1). When it will help, try to increase these closer to original values until it start to do the same as before and set the nearest lower value that worked. When it will not help, set both back to original values.
Some servers have limit for number of connections from one IP address and when there is too many connections from one address (often 4 connections, sometimes 2 connections), they will slow speed for all connections for that address or even disconnect the connection that was established as first. The second situation causes permanent connecting and disconnecting and radically slows the connection. When you can set that addon that you are using directly, do it instead of setting firefox network values.