Type about:addons<enter> in the address bar to open your Add-ons Manager.
Hot key; (<Control> [Mac=<Command>] <Shift> A)

In the Add-ons Manager, on the left, select Extensions.
Remove “ Yahoo “ Anything.

Also remove SafeSearch, SearchAssist Incognito, FileConverter Addon, SearchLock, DiscreteSearch, WebSearch, Findwide Search Engine, securedsearch

Now do the same in the computer’s Programs Folder.

Windows: Start > Control Panel > Uninstall Programs.
Mac: Open the Applications folder.
Linux: Check your user manual.

Thanks for your reply but I have nothing labeled Yahoo or any of those other things listed in my Extensions or my Applications folder. This is very insidious! And Firefox should do something about it!

It’s very sad, but many software downloaders/ installers will trick you into installing not only their program, but other programs as well.

You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print.

You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does.

From now on, everyone needs to Use The Manual Option to put a stop to this.

Note that these programs can also change browser/computer settings.

You may have ad/mal-ware. 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. Each works differently.
If one program misses something, another may pick it up.

I do scan for mal-ware. Nothing comes up. There should be a sanction against Yahoo. Meanwhile I've decamped to Chrome. So if Mozilla doesn't want to fix this problem they just don't care about losing users.

Hi eshoshkes, if the address bar drop-down says "Search with Google" but after you submit the search your results show up on a different site, there are only so many ways that kind of hijack could be accomplished.

Try disabling all your extensions by testing in Firefox's Safe Mode. Should only take a couple minutes to see whether that makes a difference:

If Firefox is not running: Hold down the option/alt key when starting Firefox. (On Windows, hold down the Shift key instead of the option/alt key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

I do scan for mal-ware. Nothing comes up. There should be a sanction against Yahoo. Meanwhile I've decamped to Chrome. So if Mozilla doesn't want to fix this problem they just don't care about losing users.

Besides malware and any software you installed or updated recently, it could be due to an Extension you installed or updated recently in Firefox. Besides Safe Mode you could disable any extensions that was installed or updated recently first one at a time to rule each out.

A lesser possibility now days on what you may be seeing. It used to be a thing in past that some ISP's would send you to their own custom search page that could be using say Yahoo when you say mistype a url.

The desktop Firefox web browser for Windows, Mac OSX, Linux has not come with anything Yahoo related since Firefox as of 57.0 in Nov 2017 was when Mozilla went back to using Google as the default on U.S., Canada, Hong Kong and Taiwan because they needed a option to replace Yahoo in those areas that had Yahoo as default. Reasons for moving away from Yahoo is in https://support.mozilla.org/en-US/forums/contributors/712925

Hi Jscher2000 - Thanks for the suggestions - I will give that a try when I have a minute. Hi James - Thanks for your response. Looks like an Extension may the culprit. I'll post again if Jscher2000's solution works.