https://support.mozilla.org/en-US/questions/857246

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. *http://kb.mozillazine.org/Profile_folder_-_Firefox You can use this button to go to the Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder 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).