Thunderbird is a free, open-source program and it costs nothing to download, install, or use. When you download it from the official web site https://www.thunderbird.net/ you will be presented with a request to donate, but it is not required and the installer will continue to download. The request screen can be ignored or dismissed by clicking the X in the upper right corner, as you can see in the attached image.

See also https://support.mozilla.org/en-US/kb/thunderbird-faq#w_is-thunderbird-free

Is Thunderbird free? Yes! Thunderbird is open source software. Anyone can download and use the program for free, and view and modify the source code under the terms of the license.

The asking for donation is optional and is not a requirement in order to download.

Check the downloads list in your Firefox web browser as you may have downloaded the Thunderbird setup in background.