While you are at it (terminal screen), type the following command:
If you get something like this:
PING 184.108.40.206 (220.127.116.11) 56(84) bytes of data.
64 bytes from 18.104.22.168: icmp_req=1 ttl=42 time=77.2 ms
64 bytes from 22.214.171.124: icmp_req=2 ttl=42 time=78.4 ms
64 bytes from 126.96.36.199: icmp_req=3 ttl=42 time=76.4 ms
64 bytes from 188.8.131.52: icmp_req=4 ttl=42 time=77.1 ms
64 bytes from 184.108.40.206: icmp_req=5 ttl=42 time=75.8 ms
64 bytes from 220.127.116.11: icmp_req=6 ttl=42 time=76.3 ms
--- 18.104.22.168 ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5007ms
rtt min/avg/max/mdev = 75.846/76.910/78.425/0.873 ms
it's a DNS problem. This means you got Internet connection, but your DNS entries aren't correct or not set, that means Firefox won't be able to resolve human readable addresses such as "www.google.com", nor will apt-get or synaptic be able to download packages etc.