I assume you've tried the usual:

(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

What happens when you try to change it?

• Firefox doesn't save the change? Look in the Windows Control Panel, Add/Remove Programs, for something named Search Protect. If you find it, click the "Installed on" column heading to sort by date and see what else was installed at the same time. This is stuff you cannot trust.

• Firefox saves the change, but if you use Ctrl+t your setting is ignored? You have an extension that is overriding Firefox's normal behavior. More on that below.

• Firefox saves the change, and it works until you exit Firefox, then the unwanted page is back the next time you start up? Check for a user.js file in your settings. This file is read at startup and overrides your previous session settings. This article has the steps to track it down and remove it: How to fix preferences that won't save.
  
  Note that some utility software like Advanced Systemcare (Surfing Protection), have features to block changes to browser settings, which can lock in unwanted settings. Turn off these features in order to finish your changes.

To review, disable, and remove extension, try the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then disable everything you don't plan to use in the next 24 hours and see whether than stops the pain.

If you haven't already removed Whitesmoke from other parts of your system, AdwCleaner can remove it. See:

• https://support.mozilla.org/questions/960297#answer-439717
• More scanners: Troubleshoot Firefox issues caused by malware