Please can you post an example URL when it takes you to Yahoo?

There are two possibilities for the issue:

• An add-on has interacted in a way that it shouldn't.
• Some third party software has modified your default search engine.

For add-ons, you can try disabling any add-ons that you have installed, and see if that helps the issue. If you do find one, please can you let us know the title of it (and preferably keep it installed but disabled, in case we need more info).

If it doesn't appear to be an add-on, please check your search settings, and see if there's anything unusual there - like a second engine pretending to be "Google". You might also want to check your computer for malware (e.g. I've heard malwarebytes is good for this), but I think I'm leaning towards an add-on causing the issue.

The Firefox web browser has not come with a Yahoo search engine since the Firefox 57.0 Release almost eight years ago as you can see in /forums/contributors/712925

I agree the Yahoo you are seeing was likely added in some way. An Extension you have installed or updated recently may have added the Yahoo search or some external software you have installed or updated recently has perhaps added this Yahoo search.