Please open Firefox and type into address bar about:config
if is the first time you are unsing about:config on firefox, a warning windows come in front of you please confirm the "promise".
Then search ( type the name into the "filter row" field ) for the firefox config param named network.online
double click on the filtered row and set it to true
quit and restart firefox
Hope this helps
Disable and stop Network Manager from services(System->Administration->Services, in Services configuration page stop and disable Network Manager).
to true in about:config
open /etc/network/interfaces and make look like this:
iface lo inet loopback