Install this extension - https://addons.mozilla.org/en-US/firefox/addon/searchreset/ - and see if that is corrected - back to the about:newtab page.

Search Reset will "do its thing" and then disappear. About this Add-on on that page lists what will be reset to the default values = resets browser.newtab.url to its default value

If you prefer to change the hidden setting manually, here's how you can change your new tab page:

(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 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. Fixed?

If problems persist, some potential diagnoses and remedies:

If Firefox won't let you edit this setting: you may have something called SearchProtect on your system. This needs to be removed from the Windows Control Panel.

If Firefox lets you save your change but ignores it: one of your extensions may be overriding it. You can review, disable, and/or remove extensions on the add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right and disable (or remove) anything unknown.

If the change works during your session, but at the next startup is back to the unwanted page: you might have a user.js file in your personal Firefox settings folder (your Firefox profile folder). This article describes how to track down and remove the file: How to fix preferences that won't save.

Any luck?