Do you see the faded icon on the upper right side that shows/hides the thumbnails, or do you get a completely different page?
If it's a different page, it may have been changed by an add-on. If the following change doesn't stick, then I would check for that possibility, as well as checking this article: How to fix preferences that won't save.
(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 search box that appears above the list, type or paste newtab and pause while the list is filtered
(3) Double-click the browser.newtab.url preference and enter the desired value:
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. Fixed?