I have a profile that is several years old and that fits comfortably like a broken in pair of shoes. :) I know at some point in my history, I removed several different search engines that I did not use and didn't want to see in the old UI's dropdown of available engines. Yahoo was one of the ones that I removed in that manner. Here is a one minute screencapture of the behavior I was seeing: http://www.screencast.com/t/Dg5EgudpcVD A textual summary follows: After the announcement of the new Yahoo search deal, I decided I wanted to try it out, but when I went into the search settings dialog, it wasn't listed. I tried clicking the link to add more engines which takes me to AMO, but there is no Yahoo! option in those results other than the Yahoo! Toolbar which is definitely not what I'm looking for. I also tried visiting yahoo.com and search.yahoo.com and checking the dropdown under the search box's magnifying glass to see if there was an option to add it that way but there wasn't. I tried a new clean profile and it showed up fine, so I figured there had to be some obvious difference between the profiles. After finding nothing useful in about:config, I diffed the two profile directories and I came across search.json. In my personal profile, there were several engines such as Amazon, eBay, Twitter, Bing, and Yahoo that all had the _hidden attribute set to either true or 1. When I tried setting that to 0, they then appeared in the new search settings UI and I was able to set Yahoo as my default. It seems to me this could be a problem that would impact adoption of the new Yahoo default depending on how many other people have previously removed/hidden those engines in the same way I did. It would probably be best to add some sort of code to the search settings page to allow recovery of those hidden engines. STEPS TO REPRODUCE (tested with Firefox Nightly 37.0a1 2014-12-11): 1. Change or verify the default search engine is set to Google and Yahoo is listed as a choice 2. Close Firefox 3. Open the Firefox Profile directory 4. Edit the search.json file 5. Find the Yahoo engine attributes in the JSON 6. Find the _hidden attribute for the Yahoo engine 7. Change the _hidden attribute to true or 1. 8. Save search.json 9. Open Firefox 10. Try to select Yahoo as the search default or to add it via searching addons.mozilla.org or yahoo.com Note that while these STR involve editing a file, this change used to be possible through the Firefox UI. I don't remember what that looked like though. :/

Quick clarification as suggested by someone on #firefox: Since the Yahoo engine *is* in search.json albeit with _hidden:1, if you visit yahoo.com, the option to add the site as a search engine is not shown. At the very least, adding an engine from a webpage should probably either disregard the _hidden attribute or cause the attribute to be reset to false since it is likely what the user is intending if they are actually on the page and trying to do something with search.

The core issue here is the inability to restore default engines in 34+, which is covered by the patch in bug 1106559 (we will land it in Firefox 35). The related issue of not being able to override a hidden default engine is covered by bug 353056.