If searches are done by Bing despite Google being set as the default, it could be due to a browser extension, malware, or incorrect settings. Check your default search engine settings, disable suspicious extensions, and run a malware scan.

This is on my Firefox app on my Android phone. Happens when I use the Google or DuckDuckGo search, I've disabled all other search engines. No extensions that I know of. I have uninstalled the Firefox app and reinstalled it.

Do you use "Microsoft SwiftKey deep search"? Other users have reported something similar with that enabled.

I use MS Swiftkey but not deep search

Is there any way you could record a screencast of it happening? Do you have any extensions installed in Firefox?

Also, it would be really helpful if you could paste the URL that is used to search Bing, so we can determine if it is our Bing or someone else hijacking the search.

If it is just showing the search term, you can click a link and then click the back button and it will show the full URL.