The exact reason for the Yahoo search engine being removed is unknown at the moment however Mozilla and Yahoo has been in a legal battle since early December. https://blog.mozilla.org/blog/2017/12/05/mozilla-files-cross-complaint-against-yahoo-holdings-and-oath/ You can install a Yahoo search engine from http://mycroftproject.com/yahoo-search-plugins.html (section 15 may be best). Also https://www.userchrome.org/yahooxml.html

Yahoo is being dropped from the built-in search engines, but you can add a new Yahoo search plugin directly from their site. When you are on a Yahoo search results page, for example, [https://search.yahoo.com/search?p=firefox https://search.yahoo.com/search?p=firefox], you will see a little green circle with a white + on the magnifying glass on Firefox's Search Bar. Click that, then use the '''Add "Get the best of the web with Y..."''' button on the drop-down to add Yahoo back. <center><img src="https://prod-cdn.sumo.mozilla.net/uploads/images/2018-03-09-13-54-09-493d1f.png"></center> ---- If you do not normally display the Search Bar: You can add the bar temporarily using Customize. See: [[Customize Firefox controls, buttons and toolbars]] If you prefer not to customize, two alternatives: (A) I posted a copy of the search plugin on the Add-ons site: https://addons.mozilla.org/firefox/addon/yahoo-com-search/ ''Right now, there's a "no go" mouse cursor when you hover over the "+ Add to Firefox" button. It's still clickable, so I don't know why it does that.'' (B) I created a page with a link you can use to load the search plugin: https://www.userchrome.org/yahooxml.html