In Firefox > open a new tab > close all other tabs > In the new tab's Address bar > type Google and press Enter >
With the google search page open, go to the Settings Menu Page > Home Page > click on Use Current Page.
If the Google search page doesn't stay as default Home page/search engine, the Add page may be an Adware... download AdwCleaner to remove it.
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/toolslib.net/downloads/viewdownload/1-adwcleaner/