Sounds like you've got malware : search.yahoo.com is a browser hijacker. And if that's what you're dealing with :

Go to the 3-bar menu (or   'Tools') => Addons => Extensions and look for any unfamiliar or suspicious looking extensions. When you find any : remove them.

However :   most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article :
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners.
They all work differently   -   what one program doesn't pick up, the other might.

Especially for Mac : https://www.malwarebytes.com/mac/

Also see : https://www.2-spyware.com/remove-search-yahoo-com-redirect.html (you'll have to scroll down to get to the relevant part for Mac OS)

As a footnote, be on the lookout for ANY extensions whose name includes the word search. For example:

• SafeSearch Incognito
• SearchAssist Incognito
• File Convertor and Search Addon

The key thing to remember when you open the Add-ons page and click Extensions in the left column, is: what you see on the right side does NOT include extensions that Firefox installs on its own. Everything listed here is your choice (and your responsibility) to manage. So don't assume anything is innocent; be skeptical!