You can check the browser.newtab.url pref on the about:config page and to make sure that this pref is set to about:newtab.

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

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

The browser.newtabpage.enabled pref should be set to true to use the page set via browser.newtab.url as the new tab page.

I tried that but it didn't seem to work.

If updating the browser.newtab.url preference doesn't work, here are 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?