Operated by MZLA Technologies Corporation, a subsidiary of the Mozilla Foundation, Thunderbird is an independent, community-driven project that is managed and overseen by the Thunderbird Council, which is elected by the Thunderbird community.

Source: https://en.m.wikipedia.org/wiki/Mozilla_Thunderbird

Yes, Gnome extensions shall be impacted once again for sure. But, at least that is not a deal breaker for me.

Let me put it in this way – if Firefox were really a spyware, TOR browser won’t be based on Firefox.