Yandex search has been removed due to russian war crimes.

See release notes.

from https://www.mozilla.org/firefox/98.0.1/releasenotes/

Yandex and Mail.ru have been removed as optional search providers in the drop-down search menu in Firefox. If you previously installed a customized version of Firefox with Yandex or Mail.ru, offered through partner distribution channels, this release removes those customizations, including add-ons and default bookmarks. Where applicable, your browser will revert back to default settings, as offered by Mozilla. All other releases of Firefox remain unaffected by the change.

You can update and/or add search engines at any time in Settings. Click the Menu button, select Settings and click Search.

You can also add Yandex search if you go to Yandex site and add it to search bar or you can go to https://mycroftproject.com/search-engines.html?name=Yandex

https://support.mozilla.org/en-US/kb/search-engine-removal https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox

If you prefer using Yandex -- or any other search engine not included in Firefox -- you can try adding it yourself using these steps:

(1) Go the the search engine home page (2) Right-click the address bar (3) If there is an item at the bottom such as Add "Yandex" click that to add it to the other one-click icons