1. Type about:config - it will show an alert if you are using latest version of Firefox, go ahead

2. In the filter type network.http

3. Modify the following values:

  network.http.max-connections to 64 (can try with different values)

  network.http.max-connections-per-server to 32 (can try with different values)

  network.http.max-persistent-connections-per-proxy to 16 (can try with different values)

  network.http.max-persistent-connections-per-server to 8 (can try with different values)

Try to find optimal values. Slow speed net users are advised to use small numbers.

Read about these values

Network.http.max-connections

Network.http.max-connections-per-server

Network.http.max-persistent-connections-per-proxy

Network.http.max-persistent-connections-per-server