Which firewall are you using?

Are you using a proxy?

Please update to Firefox 18.0.1 (released today)

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

I have Mcaffe antivirus and Microsoft security essentials. I used Enum process to check which firewall is blocking it and I got replie like below by enum:-

                            Windows Firewall is on

antispy process from Microsoft Windows Defender detected

I don't know from where I have to remove it, it will be great ifyou can give me steps to do for making my firefox work.

Does it work if you boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test?