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Go to Settings > Search and change it.
Why? probably because you have used Yahoo and they like you and want you in their life.
Perhaps an extension you have installed or updated in Firefox has added a Yahoo search engine.
The desktop Firefox web browser has not come with a Yahoo search engine as one of the defaults for any locale since Firefox 57.0 as you can see a discussion about this at https://support.mozilla.org/en-US/forums/contributors/712925
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