When did this hijack by yahoo begin? Soon after a software download?

Sounds like you're experiencing a bundled 3rd party software from Spigot. It's
an adware and is found in the Browser Extensions.

The free version of Malwarebytes should detect browser extensions by Spigot and gives you the option of deleting them.

Here you go.

• Click on the Start button (or press the Windows key) to open the Start menu, click on the Settings at the top.
• Click on App & features on the left menu.
• On the right side, locate Yahoo Toolbar and click it, then click on the Uninstall button.
• Click on Uninstall to confirm.

Jasdip, it's adware, not a chosen search engine. Most likely installed with another download.

It's adware, so it may not show up in the Control Panel. If it does, it's named Browser Extensions. Be aware that uninstall may not remove the engine (may remain in registry) and it may reappear soon after a reboot. Adware products are persistent, so it's best to use adware removers.

I agree with Michael. Download Malwarebytes to get rid of the adware. I did it and it worked but then I had to delete Malwarebytes because was interfering with the Windows security.

I actually prefer Malwarebytes to Windows Security. When I was attacked by ransomware, neither Windows Security nor the free version of Malwarebytes stopped the loading of the ransomware. Once I went through the hassle of deleting everything that was encrypted by the ransomware, I purchased the Malwarebytes Premium. It keeps my computer free from all kind of bad things. Am not impressed with Windows Security at all.

"I've uninstalled chromium and other programs on my computer but they keep coming back."

A legit Chromium installation will not do that.

Perhaps a rouge copy of Chromium came bundled with another piece of software?

I suspect most Windows users choose Google Chrome over Chromium. (Just a guess)

Chromium is perfectly legit but only if you deliberately installed it. But if it came bundled with another piece of software and just showed up unexpected, it's probably not.

If you do think you may have a bad copy of Chromium, there's detailed removal instructions on bleepingcomputer.com

It of course includes Malwarebytes, but also a few other malware removal tools in what might be called a shotgun approach.
Is each step necessary? I don't know but it's so comprehensive it should .

There's a similar question concerning Bing search engine here: https://www.gardenweb.com/discussions/5729293/bing-search-engine#n=12

It's probably unrelated to your issue but if you read through it you'll see how a screenshot can sometimes save misunderstanding.

I'm not hinting. ;)

Yayagal, I did that half a million times and it's still there.

You can't get rid of adware via the control panel. It's got built in immunity. LOL

Malwarebytes didn't get rid of it.

everytime I click on the Mozilla icon the Yahoo search comes up. I like Mozilla because it has 5 or 6 of the most used places I go to, don';t have to type anything in the search box.

Thanks for the link Richard to remove Bing...it helped me find what I needed to. Not exactly what I needed but found the way to change my home page...which for some reason had been changed to Yahoo.

I would say there's more going on in the background than you're aware of. Good luck.