There's a hidden setting for new tabs. It is quick to change, even though it takes a long time to explain it.

(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?

Two gotchas:

• If Firefox ignores the setting, one of your extensions may be overriding it. You can review, disable, and/or remove extensions on the add-ons page:
  
  orange Firefox button (or Tools menu) > Add-ons > in the left column click Extensions

• If the change works but at the next startup is back to Google, 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?

If Firefox literally refuses to take your change when you click OK, there is another possibility, which is a lock file.

Is the preference shown in italic type and indicate that it is locked? If so, the following long thread has a discussion about the mozilla.cfg file (note: do not simply delete the file, as Firefox will not be able to start):

• I'm trying to change my new tab, and I've already reset Firefox as well as going to about:config, and using search reset. - see especially cor-el's suggestion to do a "clean reinstall" (this replaces the program files folder without affecting your settings)

To clarify that last suggestion about a "clean reinstall", do NOT uninstall Firefox first. Instead, rename the program folder and then reinstall Firefox. In this process, the Firefox installer should recognize and automatically connect with your existing profiles.

(Still, it might be a good idea to make a backup. This article has suggestions: Back up and restore information in Firefox profiles.)

You can download the installer for Firefox 27.0.1 from here:

https://www.mozilla.org/firefox/all/ (scroll down to your language)

Then after exiting Firefox, rename the folder

C:\Program Files (x86)\Mozilla Firefox

something like

C:\Program Files (x86)\OldFirefox

And run the installer.