This is related to the behavior of tab instead of search engine. Do the following and you won't see Google as a Search Engine again.

1. In the Location bar, type about:config and press Enter.

(The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. )

1. Type browser.newtab.url in the search box.
2. Double-click the browser.newtab.url preference and change the url from about:home to about:blank. Alternately, you can change it to about:newtab for the Firefox's new tabbed homepage.
3. Click OK and close the about:config tab.

Hope this helps.

about:home isn't used for new tab by default.

The default for the browser.newtab.url preference is about:newtab. The 3x3 grid of previously viewed webpages can be hidden by using the "Grid" button in the upper-right corner; but as of Firefox 31 a "search container" appears, which isn't hidden via the "Grid" button.

Changing the browser.newtab.url preference to about:blank should give you a completely blank page when opening a new tab.