While you are at it (terminal screen), type the following command:
ping 8.8.8.8
If you get something like this:
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_req=1 ttl=42 time=77.2 ms
64 bytes from 8.8.8.8: icmp_req=2 ttl=42 time=78.4 ms
64 bytes from 8.8.8.8: icmp_req=3 ttl=42 time=76.4 ms
64 bytes from 8.8.8.8: icmp_req=4 ttl=42 time=77.1 ms
64 bytes from 8.8.8.8: icmp_req=5 ttl=42 time=75.8 ms
64 bytes from 8.8.8.8: icmp_req=6 ttl=42 time=76.3 ms
^C
--- 8.8.8.8 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.