These instructions were taken from mozilla's support page here
Just go through each one and see if it fixes the problem
While Firefox will work out of the box for most users, some users will have to configure Firefox for it to be able to access the Internet. Firefox may give one of the following errors when it's not properly configured:
* The proxy server is refusing connections
* Server not found
Verify that your connection settings in Firefox are correct. You can use the settings of a program that works as a reference. For example, if Internet Explorer works, open Internet Explorer's dialog to see the correct settings for your computer.
1. Open Internet Explorer.
2. In the Tools menu, select Internet Options.
3. Click the Connections tab.
4. Click LAN settings.
Then, compare the settings to those in Firefox.
1. Open the Firefox options panel.
* (Windows) At the top of the Firefox window, click on the Tools menu, and select Options....
* (Mac) On the menu bar, click on the Firefox menu, and select Preferences....
* (Linux) At the top of the Firefox window, click on the Edit menu, and select Preferences....
2. In the Advanced panel, click the Network tab. To review your Firefox connection settings, click Settings....
3. Make any changes to your Firefox settings to match those you found in your working installation of Internet Explorer.
After reconfiguring Firefox, close it and re-open it and try to access a web site.
Connection settings not remembered
If you're still having problems, check the connections settings again. If you notice that your changes haven't been remembered, you may have some software on your computer that is overriding your settings.
One program known to override connection settings is Hide IP. To remove the override:
1. Open the profile folder.
2. Open the user.js file for editing.
* If there is no user.js file, you don't have any overriding settings, so this isn't your problem.
3. Remove any lines that have the word "proxy" in them.
4. Save the file.
5. Restart Firefox and try to change your settings again.
Firefox is in offline mode
Firefox has an offline mode where it won't try to use the Internet. If your Firefox is in offline mode, it will show Offline mode when you try to use web sites. To turn off offline mode:
1. Open the File menu.
2. If there is a check mark beside Work Offline, click Work Offline to remove the check mark.
* If there's no check mark, Firefox is not in offline mode.
Some security software, for example firewalls, can block Firefox from accessing the Internet. This is especially common after a Firefox upgrade. When security software blocks Firefox, Firefox may give any of the following errors:
* Unable to connect
* Server not found
For information on how to configure security software to allow Firefox access, see Firewalls.
Older versions of Panda Anti-virus can cause connection problems.
Note: Information on this issue is no longer available on the Pandasoft Web site. You should upgrade your version of the software, disable it, or replace it with a different anti-virus program.
Internet Protocol version 6 (IPv6) can cause connection problems on some Internet connections. Firefox uses IPv6 by default. Disable IPv6 to see if this solves your connection issues:
1. In the Firefox Location bar, type about:config and press Enter.
2. In the displayed list, type network.dns.disableIPv6 in the Filter textbox.
3. Find the entry for network.dns.disableIPv6.
4. If the value is false, right-click it and select Toggle.
* If the value is true, IPv6 is already disabled, so it isn't causing your problem.
5. Close Firefox, start it again, and try to reach a web site.
If disabling IPv6 doesn't resolve the problem, re-enable IPv6 by setting the preference back to false.