For a while, the Thunderbird default search engine was pegged to Bing. There was no easy way to change it. A marketer came to the bugzilla to explain they would not fix it for business reasons.
Which seems backwards because before Chrome, Google would pay Mozilla pretty large amounts of money [0] just to have their search as an easily-changeable default.
They diversified. I Think they provide Yahoo search by default most places, Yandex in Russia, and some other one in China.
Yahoo! is the default for U.S.A. Yandex for Russia, Baidu for China. All the others default to Google. And yes, you can always change the default search engine to your preferred one in the preferences.