Mozilla Suite : Issues : Connect
From MozillaZine Knowledge Base
Mozilla will not connect to any site
If you are using a firewall - check the settings so that Mozilla is allowed to connect.
Note: Even a deactivated firewall (software) can block traffic
If you are using a proxy - check the settings. If things are working with Internet Explorer or any other browsers - transfer the settings from there to Mozilla.
When Mozilla access a secure site or mail server, it needs to access Personal Security Manager (PSM), but some firewalls mistakens the internal communication as an attempt to set up a local server. If you are prompt about this, you may safely grant Mozilla access.