From the donation page:

> Contributions go to MZLA Technologies Corporation, a California corporation wholly owned by the Mozilla Foundation. Funds will be reserved for use in the Thunderbird project. Contributions are not tax-deductible as charitable contributions.

I thought that too, I was a bit confused by the fact Mozilla are still operating the donations in the OP, which is why I mention Mozilla specifically. Definitely s/Mozilla/Thunderbird devs/ if that's wrong though.