First thing to do in such cases is to check the network settings for Firefox. In 90 % of the situations encountered, the issue is directly influenced by the proxy server. Go to Firefox > Tools > Options > Advanced > Network and make sure there is no proxy set up. Check No Proxy then attempt to browse and download again. Additionally, turn off your Firewall or allow Firefox to communicate with Internet through Firewall. Go to Start > Control panel > System & Security > Windows Firewall.