At the top of your Firefox page, go to EDIT , then scroll down to preferences, then under GENERAL, it will say "When Firefox starts show my homepage. ,type in any web page you want or choose from use current pages ,use bookmarks , or return to default

There's a hidden setting for what appears on new tabs. You can change that here:

(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 the unwanted site, 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?

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

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