hi, thank you for the report - there seems to be an increase in malware addons recently that force yahoo search upon users against their will. could you enter about:support into the address bar of firefox, then copy the text contents of that page, paste it to a site like https://dpaste.com/ and provide the link here in a reply?

thank you

Firefox since 57.0 has not had any Yahoo search so a Firefox download or updated from Mozilla was not the cause.

I agree that it was likely due to a extension that was installed or updated recently.