I have been researching this all day and have yet seen a solution.
I saw on that was suppose to work with an about:config setting however it wasn't available so added it and it didn't work even after restarting Firefox. It was over on the FVD Speed Dial - Chrome forum however Chrome doesn't have about:config and the closest thing that Chrome has is about:flags.
I don't even have bing as installed search engine on my Firefox.
I saw where some said modify the hosts file so that it redirects to Google. It did redirect however Firefox saw a SSL certificate issue and blocked access to Google. I looked at the config stuff for FVD Speed Dial to see if I could figure out where it was calling to Bing and could not find it.
This is been go on for over 6 years. It is time to fix it, A simple solution is to use the default search engine for the browser. I have used an extension that had weather and search bar as well as a random image and a quote. It uses whatever the default search engine is for Chrome. If I had it set to the search engine to Duck Duck Go, the extension used it. Likewise if I had the default search engine set for The Internet Archive, it use it. I like usually use either Google or The Internet Archive most of the time.
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