Bing isn't the default search engine in Firefox, so your search engine would never be changed to Bing.

It's probably the case that you have an add-on or malware that is injecting Bing. I'd suggest downloading Malwarebytes (www.malwarebytes.com) and running a full scan to find anything on your computer.

It's not really "invading your privacy" to add a search engine to Firefox. It's likely just happening back you have the Automatically update search engines option enabled in your settings.

If it's constantly being reset as the default search engine, this is probably caused by an add-on or another program that you have installed on your computer. There are some security programs known for changing the default search engine.

Hope this helps.