It is possibly some form of adware or malware. Firefox does now stipulate that most Extensions must be signed, that helps prevent some abuse, however an add-on to search using Yahoo, or any other Search Engine or portal is allowable if that is its purpose and description. Not much we can do especially if you do not know what the add-on was or where you got it from. You did however post https://support.mozilla.org/en-US/questions/1144872#answer-942840 <blockquote>I think this junk came with McAfee add-on I installed. But honestly, where was that warning when I installed McAfee? Shouldn't I know about more stuff added to my browser beforehand? This is frustrating. Preventive options don't exist, I guess.</blockquote> Even genuine websites encourage you to install money making bloatware by using opt out and small print instead of clear opt ins. Adobe often offers addtional software that I never need or want. If you end up on less scrupulous or malware sites you may just get malware installed with little or no warning. Please see * [[How to stay safe on the web]] * [[How does built-in Phishing and Malware Protection work?]] * [[How does built-in Phishing and Malware Protection work?]] If you have an English US build of Firefox - probably the one most English speakers install - that uses Yahoo as it default search engine, but has others including Google built in, and it is easy to change search engines temporarily or permanently, or add almost any search Engine of your choice. * '''[[Change your default search settings in Firefox]]'''

In About:config, I found this line; browser.search.defaultenginename.US The full line is; data:text/plain,browser.search.defaultenginename.US=yahoo Change the final word after the '=' to google, and you should be fine