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?

You can check these prefs on the about:config page and reset them to the default via the right-click context menu in case they appear bold (user set).

• browser.newtabpage.enabled
• browser.newtab.url

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

You can use the SearchReset extension to reset some preferences to the default values.

• https://addons.mozilla.org/firefox/addon/searchreset/

Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).