Mozilla the non-profit owns the for-profit corp who owns Thunderbird.
In their own words from the footer of https://www.thunderbird.net:
> Thunderbird is now part of MZLA Technologies Corporation, a wholly owned subsidiary of Mozilla Foundation.
> Who makes Thunderbird?
> Thunderbird is developed, tested, translated and supported largely by group of dedicated volunteers, plus paid staff. Thunderbird is an independent, community driven project. Therefore its paid staff, budget and fundraising are entirely managed and overseen by the Thunderbird Council, which is elected by the Thunderbird Community. Thunderbird development is made possible by funds donated by the Thunderbird community. (Mozilla Corporation, the makers of Firefox, and Mozilla Messaging no longer develop Thunderbird. But Mozilla still supports Thunderbird by hosting many of the Thunderbird resources.)
[1] https://support.mozilla.org/en-US/kb/thunderbird-faq#w_who-m...