You can check for issues caused by recently installed suspicious or unknown extensions.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I did everything you suggested. I went back to using the Default Firefox Theme and it was working for several days. Today, I switched back to my preferred theme which has a "Safari" like appearance, and Firefox switched back to Yahoo.

I am back to using the default theme, which I don't like, so if you have any thoughts/suggestions please let me know.

Thanks - Ted

What Extensions do you have installed in Firefox?

Firefox from mozilla.org (www.mozilla.org/firefox/all/) no longer comes with Yahoo as one of the search engine defaults for any locale. So that is not the reason.

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.

https://blog.mozilla.org/blog/2017/11/14/firefox-features-google-as-default-search-provider-in-the-u-s-canada-hong-kong-and-taiwan/

Reasons for moving away from Yahoo is in https://support.mozilla.org/en-US/forums/contributors/712925

You can rename/remove search.json.mozlz4 (and possible search.json, search-metadata.json, search.sqlite) in the Firefox profile folder with Firefox closed to rebuild the search engines registry.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory/Folder:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

Firefox will rebuild the search.json.mozlz4 file from the builtin search engines.

These are the extensions I have enabled:

1Password Adobe Acrobat Grammarly for Firefox Honey Privacy Badger

I have the same prob as TW, for Mac Firefox 72. Without notice, Firefox will change my default search engine on its own, from Google to Yahoo.....??

I don't understand what removing/changing the search.json.mozlz4 file will do as cor-el suggested, as it will just rebuild to the same profile, right?? That won't really solve the prob of its changing the default search engine on its own.....??

Of course, it changes the default search engine whenever I download an update, but it also changes the default search engine at odd times--as I said, on its own......??

Thanks! --Jackie

@ TW and jyoung1 :

The problem may be caused by malware; see these articles :

(you'll have to scroll down to the for Firefox users relevant part)

https://geeksadvice.com/remove-search-yahoo-com-redirect/

https://macsecurity.net/view/259-yahoo-redirect-virus-mac

Also see : https://discussions.apple.com/thread/8352212

Running all malware scanners, that are mentioned in this article
might be a good idea.

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