There's a hidden setting for what appears on new tabs, which you can edit as follows:
(1) In a new tab, type or paste about:config in the address bar and press Enter.
(2) In the search box above the list, type or paste newtab and pause while the list is filtered
(3) Double-click the browser.newtab.url preference and enter your preferred page:
Page thumbnails (default) => about:newtab
Blank tab => about:blank
Built-in Firefox home page => about:home
Any other page => full URL to the page
Press Ctrl+t to open a new tab and verify that it worked.