Hi  ! Yahoo Search is a legitimate search engine, but you may be dealing with search.yahoo.com, which is a browser hijacker, malware that you of course want to get rid of.

Go to the 3-bar menu (or   'Tools') => Addons => Extensions and look for any unfamiliar or suspicious looking extensions, like e.g. :

SafeSearch Incognito SearchAssist Incognito Search for Fire Fox 1.6 SafeSearch ch SafeSearch net File Convertor and Search Addon

When you find any : remove them.

Do the same in Windows' Control Panel => Programs.

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.

Also see : https://malwaretips.com/blogs/yahoo-toolbar-removal/

I've done these checks and Yahoo is STILL defaulting when I have not selected that - my Firefox default is Google!!! I use NOD32 by Eset and it has not detected any malware. How do I change this??? Uninstall and reinstall Firefox perhaps? Thanks, John

This could be an issue caused by AdAware or Web Companion from Lavasoft.

You can look for a file named dsengine.js in these locations. You should only find channel-prefs.js in the "defaults\pref" location. Any file found here apart from channel-prefs.js is suspicious. You can check the content of the file in a text editor (use open with and do not double-click the file).

C:\Program Files\Mozilla Firefox\defaults\pref\
C:\Program Files (x86)\Mozilla Firefox\defaults\pref\

You can look for a file named dsengine.cfg in the main Firefox program folder.

C:\Program Files\Mozilla Firefox\
C:\Program Files (x86)\Mozilla Firefox\

Delete the dsengine.js and dsengine.cfg files when present.