If I remember correctly, default settings are good in most of cases, but if you need to change something, go to menu [B]Tools[/B] --> [B]Options[/B]
Click on [B]Advanced[/B] and then on the [B]Network[/B] tab. Finally, click on the [B]Settings[/B] button. There you can decide how Firefox connect to internet.
Check also the menu [B]File[/B]. If the option “[B]Work Offline[/B]” is selected, uncheck it.