The search container on the New Tab page isn't labeled as to which search engine will be used; the default search engine would be used there. So if you have Google set as your default search engine, Google would be used.

See this support article: https://support.mozilla.org/en-US/kb/use-search-bar-firefox

In the Search box in top line Yahoo is my search engine. When a NEWTAB opens Google is in the middle of the page with a search dialog box waiting.

Unless the Linux built versions of Firefox have changed the default search features in Firefox: the URL bar, the Search Bar, the about:home and New Tab search containers should all use the Default Search engine that the user has set.

What extensions do you have installed? Extensions can add prefs to change the New Tab page.

I use Kubuntu as downloaded. Firefox 47.0 NO add-ons. Preferences: Yahoo, Bing, Wiki (en) BTW- Google still shows as default #2 in config. Did all the changes: UNcheck Block reported web forgeries, UNcheck Block reported attack sites. Thats it.

https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox

The Linux distro provided packages of Firefox may be modified depending compared to Linux builds from www.mozilla.org/firefox/all/