This could be a feature of an add-on, in which case you would need to disable or remove that add-on. But first, you could try resetting the new tab URL as follows.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter the desired value:

(i) Page thumbnails (default)

browser.newtab.url = about:newtab

(ii) Blank tab

browser.newtab.url = about:blank

(iii) Built-in Firefox home page

browser.newtab.url = about:home

(iv) Any other page

browser.newtab.url = full URL to the page

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

If it isn't fixed or reverts back after restarting Firefox, try disabling ALL nonessential extensions here:

orange Firefox button or classic Tools menu > Add-ons > Extensions category

Then try the first procedure again. Fingers crossed!

Thanks for the great info. It really worked.

See also this article about the New Tab page (about:newtab):

• https://support.mozilla.org/kb/new-tab-page-show-hide-and-customize-top-sites