Mozilla has nothing to do with Yahoo search engine now as it was removed as one of the defaults in Firefox six years ago. /forums/contributors/712925

If you are getting a Yahoo search now then it could have been added in a recent Extension install or update or perhaps by some malware on your system. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Btw Firefox is a web browser. The DuckDuckGo, Bing, Google, Yahoo etc are Search Engines. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox

Thank you for sharing your add-ons list. I noticed that one of them is not from the Mozilla Add-ons site. Any idea where it came from or why you have it?

"name": "Browsing Safety by Safely", "version": "1.1.2", "id": "{8c9ec486-bd7b-40dd-ab49-1ca3ff452484}"

Turns out it was added to the Add-on block list four years ago (bug #1614707), so I'm surprised it was able to get or stay installed on your Firefox recently.

Maybe there is a problem with malicious add-on blocking in your Firefox, or maybe older blocked IDs fall off the list??

As a first step, definitely get rid of the hijacker.

As a second step, you probably should do some malware scans. See: Troubleshoot Firefox issues caused by malware.

As a third step, we should help you investigate whether blocklisting was somehow hacked on your Firefox.

You can check extensions.blocklist.enabled in about:config and make sure it is default true to ensure that you are safe.