How to remove onesearch.org

My default search engine in firefox has been hijacked to be Onesearch.org. Preferences still state it is Duckduckgo, but that's not what comes up. A scan with Sophos shows no problem. A previous message suggested I run adwaremedic, which is now Malwarebytes. I downloaded Malwarebytes and it says there are no problems, but onesearch.org still comes up as the search engine.

This is on a Macbook Air running OS 10.10.5

Any help will be greatly appreciated.