Did you do a clean (re)install and deleted the Firefox program folder (C:\Program Files\Mozilla Firefox\)? Did you try a new profile? *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall ---- 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 and the updater process. See: *https://support.mozilla.org/kb/Server+not+found *https://support.mozilla.org/kb/Firewalls ---- Do a malware check with some malware scanning programs on the Windows computer.<br /> You need to scan with all programs because each program detects different malware.<br /> Make sure that you update each program to get the latest version of their databases before doing a scan.<br /><br /> *http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware *http://www.superantispyware.com/ - SuperAntispyware *http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner *http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page *http://www.safer-networking.org/en/index.html - Spybot Search & Destroy *http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free See also: *"Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked