If Google disappeared from your Firefox search engines
Yahoo or some “conduit” engine may appear in the search box of Firefox contrary to what you have configured, while Google not only stopped being your default engine, but it has disappeared completely and you cannot use it anymore through the search box of Firefox.
To remedy this situation, first uninstall the program that is responsible for this hijack of your system. FireTV, for example, is a Firefox plugin known for such behavior.
After you finish the removal of the offending application, click on the icon at the left of the Firefox Search Box and choose the “Manage Search Engines” option. Remove whatever engines you want removed.
Finally, go to Mozilla and get a Google engine. Start using it immediately to become your default search engine. Enjoy!