How can I stop Firefox from redirecting me to a Yahoo search page when it gets a 404 - Page not found error?

I assume that you already tried Safe Mode? *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes Check the connection settings. *Firefox > Preferences > Advanced > Network : Connection > Settings *https://support.mozilla.org/kb/Options+window+-+Advanced+panel If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. See "Firefox connection settings": *https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can You can check for problems with preferences and possibly reset all prefs. *http://kb.mozillazine.org/Preferences_not_saved *http://kb.mozillazine.org/Resetting_preferences ---- You may have an ISP that automatically redirects you to a search page if a DNS look up fails.<br /> That takes the control from Firefox to do the normal keyword search like a Google I'm Lucky search.<br /> In such cases you need to contact your ISP and ask for a DNS server to opt-out of that feature.<br /> You can also check if there is a link on the search results page to go to your ISP and opt-out. *http://en.wikipedia.org/wiki/DNS_hijacking