I believe the distribution engines are those used in addons.mozilla.org. But honestly I do not know for sure. It is possible to manage search engines in https://support.mozilla.org/en-US/kb/use-search-bar-firefox#w_change-your...

Thanks, but that’s not really a solution to my problem – not the only one I was having with FF 40. I’m downgrading to ESR now and I fear I’m going to have to find another browser by May 2016 (when support of FF 38 will end) :o(

Any search plugin which is in the /searchplugins/ folder in the Profile folder which has the same <ShortName> as a default search engine won't be recognized or used in Firefox. The default search plugin takes precedence over a user installed search plugin when the name is the same.

The "trick" is to open the XML file for each unrecognized search engine in the /searchplugins/ folder and edited the <ShortName> to be different. I added a hyphen, as in Google- and saved the search engine XML file as google-.xml back in the /searchplugins/ folder. When I started Firefox I could see the three search engines that had "gone missing" in Firefox 40.

Plus the default search engines were moved out of the /searchplugins/ folder to keep them safe from being hijacked.

In Firefox 40+ the built-in default search engines are no longer in the "/browser/searchengines/" folder, but have moved to the "/browser/omni.ja" archive.

In Firefox you can inspect the built-in default search engines by pasting this resource URI in the location bar (modify the language code if applicable).

• resource:///chrome/en-US/locale/browser/searchplugins/

Just a note about customizing the ShortName in your modified plugins: I read in an article that you get 16 characters, so you can't get too verbose.