I mean, as I've somewhat said above, I do donate to Mozilla for a direct-but-big reason. Overall, I find their work VERY important. I acknowledge that they've never been perfect, but I've watched what they've done for 20-30 years and strongly trust that generally, they're doing good things with my money because that's what they've been doing.

Thunderbird, separate from Mozilla, I don't think has that to rely on. That does feel more like "why should I give money to this project that (for me) has been pretty mid at maintaining a popular piece of software?"