The Firefox web browser since 57.0 has not had any Yahoo search so a Firefox download or update from Mozilla.org was not the cause. The Yahoo search being removed was discussed in https://support.mozilla.org/en-US/forums/contributors/712925

It could be due to a extension that was installed or updated recently so check all of your Extensions you have installed in Firefox.

Some form of malware or hijack perhaps. If you are seeing this recently then perhaps something you installed or updated recently on Windows took control. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Thank you very much, it was an extension.

Note that the System Details list shows that you have a user.js file in the profile folder to initialize prefs# each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

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 Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data