https://www.mozilla.org/en-US/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 still add it back to search bar while on Yandex page or by https://mycroftproject.com/search-engines.html?name=yandex

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

Search engine defaults changes. Yahoo was the primary default from in 2015 to 2017 instead of Google and Mozilla and Yahoo have been in legal dispute since. In 2023 Mozilla may be looking elsewhere to replace Google again.

There is also talk that Yandex may be defaulting or going bankrupt at some point anyways.