Primarily, but there are others. Yandex and Baidu in 2020 and presumably still: https://www.theverge.com/2020/8/15/21370020/mozilla-google-f.... Others in the past, not sure if any are still active: https://www.cnet.com/news/firefox-and-bing-together-at-last/. They're currently trialling Bing too: https://www.ghacks.net/2021/09/17/firefox-experiment-is-test...