may be it is a DNS related problem? or gateway or proxy...
can you ping an internet site, let's say www.google.com
from your virtual machine? if so, it's just a config problem in mozilla (since i don't use it i can't help you in that way)
if not, try to ping the IP address of google (do a ping on your host machine to have it). If you can ping this IP, then you'll have to configure a DNS in the /etc/resolv.conf of your virtual system.
if you can't ping this IP, then you'll need to configure your network with a gateway (so then you'll be able to go out of your intranet)
both the gateway and the DNS are network specific, so i can't tell you generic values...