Your installed extensions list shows 3 dodgy add-ons, that are known Malware / Virus carrier's. '''eBay Shopping Assistant 2.4 ({A92BC493-FA5B-40a5-AFC8-6672926C9544}) '''Searchme 2.8 ([email protected]) '''Slick Savings 3.8 ({84977DE9-1026-4508-B9A7-9116BDAAB098}) Read about what each of those does to your computer. http://pcviruskiller.blogspot.com/2015/07/what-is-ebay-shopping-assistant-by.html https://malwaretips.com/blogs/remove-searchme-toolbar/ https://malwaretips.com/blogs/slick-savings-virus/ I can't help you with any Mac OSX specifics as far a removing them, but do recommend that you read this support article. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware#w_how-do-i-get-rid-of-malware In the end it is probably best that you visit one or more of the 'malware removal' fora shown at the bottom of that page. ---------------- As far as having a problem setting your own 'default' search engine. please see this support article to learn how changing the 'default' is done in Firefox 43.0. https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox But you should get rid of all the Malware first; one of those 3 extensions might have 'locked' that preference and will keep resetting it as Firefox opens until you get rid of them.