and how to remove it?

There are several extensions that allow to do an image search on various search engines, I'm not considering this as a spam function if you have installed such an extension.

• https://addons.mozilla.org/en-US/firefox/search/?type=extension&q=yandex
• Search by Image https://addons.mozilla.org/firefox/addon/search_by_image/
• Find Image In Yandex https://addons.mozilla.org/firefox/addon/yandex-search-image/

There are several extensions that allow to do an image search on various search engines, I'm not considering this as a spam function if you have installed such an extension.

• https://addons.mozilla.org/en-US/firefox/search/?type=extension&q=yandex
• Search by Image https://addons.mozilla.org/firefox/addon/search_by_image/
• Find Image In Yandex https://addons.mozilla.org/firefox/addon/yandex-search-image/

I have never installed it! Yandex is a spam, i want to figure out which extention did it and get rid from this option.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

• if you have many extensions then first enable half of the extensions to test which half has the problem.
• continue to divide the bad half that still has the issue until you find which one is causing the problem.