It's good that you donated to a project that depends on user support to continue to develop the product. But it wasn't necessary, and any mention you see about donations is merely a suggestion, not a requirement. You don't need to donate in order to download, install, or use Thunderbird. It is a free product.

Are you sure you downloaded it from the official source, and not some illegitimate or malicious site?

https://www.thunderbird.net/en-US/

The following link might be helpful: thunderbird download https://support.mozilla.org/en-US/questions/1511753

Quotation from the above link:

3. After you click "Download" button, the page will show request for donation, but you should find the downloaded file on the top right corner of your browser.

Please see the attached images in the above link.