Hi, please see - Add or remove a search engine in Firefox.

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Not sure about Win64 builds but Yandex is the default search engine in Russian locale of Releases as Yahoo is the default in en-US.

Besides installing the Russian version of Nightly channel builds you can add search engines from http://mycroftproject.com/

If you are not able to install a search plugin for Yandex from the Add-ons site or Mycroft, you could try installing one from the site (this works in Firefox 41, I haven't tested in Nightly):

Visit https://www.yandex.ru/ and notice there is a green circle with a + on it on Firefox's search bar. Click that and then find the line on the drop-down to add the site as a search engine.

I have attached a screen shot but since I can't actually read the language, I apologize if it's totally something else.

To make Yandex your default you can use the link at the bottom of the panel to the Search section of the Options page.

Success?

Regarding Yandex Elements, is the problem that Yandex has not submitted the extension to have it signed by the Add-ons team? This is something you could feel free to encourage them to do.

you can actually download elements from https://element.yandex.ru (push big yellow bar), but I'm not sure about compability with beta and nightly builds (and if it works in english if that's the case). (I suggest disabling of shopping suggestor, because this thing is quite annoying)