Sure, if you can add it, you can make it the default. Go the home page of your preferred Yandex site, click the Page Actions menu button (•••) in the address bar, and the bottom item of the menu should be to add the current site's search engine plugin to Firefox. After that, it should appear as one of the One-click icons and be available in the Default search engine drop-down to make your default. Sadly, not all search sites make it so easy to add them. ---- By the way, your Firefox identified itself as version 74, which is no longer supported. Any reason you're not updating that we might be able to help with?