Reset the browser.startup.homepage_override.mstone pref via the right-click context menu to the default on the about:config page and close and restart Firefox to restore the about:home page to its default.

• http://kb.mozillazine.org/about:config

1. Open the about:config page via the location bar
2. Type in the Filter bar: mstone
3. Right-click the browser.startup.homepage_override.mstone line and select: Reset
4. 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.

See if this works to reset that about:home search engine back to Google.

With the about:home page loaded, paste this in the Location Bar and hit Enter.

 
javascript:{localStorage["search-engine"]="{\"name\":\"Google\",\"searchUrl\":\"http://www.google.com/search?q=_searchTerms_&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a\"}";void(0);}
 

Note that the code only changes the name and URL and doesn't restore the icon and other parameters in local storage, so it is better to reset the browser.startup.homepage_override.mstone as the preferred method to reset the about:home page.