I don't know "why" but here's what I would check:

(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?

Some gotchas:

If Firefox won't let you edit this setting: you may have something called SearchProtect on your system. Remove that through 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:

"3-bar" menu button (or Tools menu) > Add-ons > in the left column click Extensions

If the change works during your session, but at the next startup is back to Bing: 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?

Bless you. So far that's worked. Now I have to see if it holds up in the restart.

Next start, no Bing. Thank goodness. Thanks, jscher2000!