Addon provides search engine by Yandex (Яndex, Яндекс) and changes design of Yandex sites:
ya.ru
yandex.ru
yandex.by
yandex.kz
yandex.eu
yandex.com
yandex.com.tr

You can use it:
1) in search bar
2) in address bar with 0 (Zero) or Я (Cyrillic) or Z prefix

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For Firefox browser identification addon uses Client ID from Yandex OpenSearch
yandex.ru/search/opensearch.xml
I am testing this now. And it seems to help reduce the appearance of "I'm not a robot" checks and captchas.

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Icon templates from source:
Flaticon.com
icons8.com

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My other addons, maybe you find them useful:
https://addons.mozilla.org/en-US/firefox/user/16521499/

Install

• License: Mozilla Public License 2.0
• Weekly downloads: 279
• Average daily users: 16441
• Rating: 3.7188/5 of 64 ratings
• Created: 2022-06-01 10:13:30
• Last updated: 2023-08-07 11:07:17
• Homepage: https://sites.google.com/view/theylive/home
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/яндекс-поиск-yandex-search/
• API: {f479fa23-7074-4c16-bca2-4fae263f7e5a}