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 *http://kb.mozillazine.org/Browser_will_not_start_up ---- Can you start Firefox in Offline mode via the Profile Manager *http://kb.mozillazine.org/Profile_Manager ---- If there is a problem with searching via the default about:home page then try to reset the search engine used on the about:home page. Reset the <b>browser.startup.homepage_override.mstone</b> pref via the right-click context menu to the default on the <b>about:config</b> page and close and restart Firefox to restore the about:home page to its default. *http://kb.mozillazine.org/about:config #Open the <b>about:config</b> page via the location bar #Type in the Filter bar: mstone #Right-click the browser.startup.homepage_override.mstone line and select: Reset #Close and restart Firefox If this didn't help then delete the delete the chromeappsstore.sqlite file in the Firefox Profile Folder and repeat the above. You can reset some preferences to the default with the SearchReset extension: *https://addons.mozilla.org/firefox/addon/searchreset/ Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).