Do you mean on the Mozilla Add-ons site? Hmm... If you are not able to install a search plugin for Yandex from the Add-ons site, you could try installing one as follows: (1) 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? (2) The following site has numerous add-on search plugins. I haven't tested any of their Yandex plugins: http://mycroftproject.com/search-engines.html?name=yandex If you scroll down to "8. Major Engines" and/or categories 10 and 11, hopefully you can find one you like.