Please Copy/Paste about:preferences#search to the Address Bar then Enter.

Please look in One Click Search Engines and see if Yahoo is listed. If yahoo is listed make your change to the search engine you wish. You can then Click Yahoo and Remove it.

If it is not there please look in your Installed Programs and see if there is a Yahoo in there, if so, uninstall it.

If no luck you can run this please : https://www.malwarebytes.com/mac/

Please let us know if this solved your issue or if need further assistance.

Could be one of your 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 -> Appearance
• 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

If it works in Safe Mode and in normal mode with all extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) disabled then try to find which extension is causing the problem by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via one of these:

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

• if you have many extensions then first enable half of the extensions to test which half has the problem.
• continue to divide the bad half that still has the issue until you find which one is causing the problem.