there are 3 scenarios that could prevent your browser from accessing the internet

• A virus attack has maimed the sockets in your computer
• A firewall is blocking Firefox's access to the internet
• The proxy server is invalid

I will go through steps to correct each of them ; preferably you should follow the steps in the order given

In the first case, log into your computer through the safe mode and scan your computer for viruses. You could use one of the Free-Software's such as Avira’s Antivir. If you do find a virus simply put it in Quarantine. Deleting  a virus can sometimes make your system very unstable. What i have found is that quarantining the infected files doesn’t seem to crash your PC.

After the scan is done. Log in as a normal user and run one of the Winsock fixing software's. You could also right click on your Network connection and hit repair Network. If your problem had been due to a virus that should fix it.

Firewall

Your Firewall presents another bottleneck. Ideally speaking you must delete all references to your browser from you Firewall software and if at all there are any references make sure that you allow complete access to the browser.

In the Windows Firewall you could add Firefox or other browser to the exceptions list if it is not there already.

In COMODOD Personal Firewall. Click on the Firewall tab and choose Network Security Policy under Advanced tab. Make sure Firefox is  not blocked.

Under Zone alarm delete all references to Firefox. That should fix any firewall related problems.

 

PROXY

If all the above steps failed then this must be your problem .Check the proxy settings. You / the virus may have probably re-routed the data through a proxy and forgotten about it(You of course!). To access the proxy settings. Go to  Tools—>Options—>Advanced—>Network—>Network settings. Choose “No Proxy”.

And lo! your browser should be working.