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/