When added to the Firefox browser, the New Tab Override extension customizes the page that shows whenever you open a new tab. Once choosing a desired search engine webpage, add its address to the extension settings. Your chosen search page will now be shown whenever you open a new tab.
When you type in the search bar the automatic URL address bar feature will no longer be accessed. You will however lose the other features of the Firefox default new tab page and be limited to the features of the search engine page.
If New Tab Override does not provide all the features you need, try searching for “new tab” within Firefox extensions for a large selection of new tab customizations.