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 ---- Create a new profile as a test to check if your current profile is causing the problems.<br /> See: * [https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile Basic Troubleshooting: Make a new profile] If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)<br /> See: * http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox