Your More system details list show that you run the latest Firefox 4.0.1 version.<br /> You can check that via Help > About Firefox and on the Help > Troubleshooting Information page. A possible cause is security software (firewall) 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 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. See: * https://support.mozilla.com/kb/Server+not+found * https://support.mozilla.com/kb/Firewalls ---- If you posted from another computer then download the full version and uninstall the currently installed version. Download a fresh Firefox copy and save the file to the desktop. * Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html * Uninstall your current Firefox version. * Do not remove personal data when you uninstall the current version. Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer. * It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling. Your bookmarks and other profile data are stored elsewhere in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder] and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.