Hello julianhanford. Unfortunatley your dealing with malware that you of course will want to get rid of. See : http://www.besttechtips.org/how-to-remove-onlinevideoconverter-com/ Go to the Firefox menu (≡) Preferences => Add-ons => Extensions and remove anything that looks unfamiliar or suspicious. (your system details only show two legit extensions, though) Next step : run malwarescans. Further information can be found in this article : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no '''Run most or all of the listed malware scanners'''. They all work differently - what one program doesn't pick up, the other might. Especially for Mac : https://www.malwarebytes.com/mac/ And : https://malwaretips.com/blogs/remove-mac-os-x-virus/ Also see : https://hotvirusmalwareremoval.com/tips-on-how-to-remove-online-video-converter-ads-quickly-mar-2018-update/ 'Hope that helped you to get rid of it !