Is it an add-on? website? . . .

Are you talking about the home page? https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Or the search engine? Type about:preferences#search<enter> in the address bar. You can select any search engine to be your default from here.

You can also remove any engine from here.

At the bottom of the Freshy page there is an "Unsubscribe" tab.

Looks like an extension that overrides the new tab page.

See also:

• https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons