Question: 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.
MacBook Air, OS X Yosemite (10.10.5)