Hello, I created a new extension that will allow you to fix your problem: https://addons.mozilla.org/en/firefox/addon/bing-to-whatever/.

In the settings of 'bing to whatever' , select Google to use it by default for searches from the Windows bar.

If everything is fine, don't hesitate to change your opinion on this extension, and give yours on the new one! If the problem persists, contact me again.