The reason "nobody questions how corporations use their money" is that in 99.9% of cases when I pay a corporation money for a product, I'm doing it not for the sake of what they can do with the money, but because otherwise I don't get to use the product, at least not legally.

If instead I donate to an open-source project, I'm not doing it in order to get access to the product; I already have that. I'm doing it because I hope they will do something with the money that I value. (Possible examples: Developing new features I like. Rewarding people who already developed features I liked. Activism for causes I approve of. Continuing to provide something that benefits everyone and not just me.)

And so I care a lot what they're going to do with the money, in a way I don't if I (say) pay money to Microsoft in exchange for the right to use Microsoft Office. Because what they're going to do with the money determines what point there is in my giving it.

Sometimes, everything the project does is stuff I think is valuable (for me or for the world). In that case I don't need to ask exactly what they're doing. Sometimes, it's obvious that what happens to the money is that it goes into the developer's pockets and they get to do what they like with it. In that case, I'll donate if the point of my donation is to reward someone who is doing something I'm glad they're doing, and probably not otherwise.

In the case of Thunderbird, it's maybe not so obvious. Probably the money will go toward implementing Thunderbird features and bug fixes, but looking at the history of Firefox I might worry that that's going to mean "AI integrations that actual users mostly don't want" or "implementing advertising to help raise funds", and I might have a variety of attitudes to those things. Or it might go toward some sort of internet activism, and again I might have a variety of attitudes to that depending on exactly what they're agitating for. Or maybe I might worry that the money will mostly end up helping to pay the salary of the CEO of Mozilla. (I don't think that's actually possible, but I can imagine situations where Mozilla wants some things done, and if they can pay for them via donations rather than using the company's money they'll do so, so that the net effect of donating is simply to increase Mozilla's profits.)

And I don't think anyone's asking for anything very burdensome in the way of transparency. Just more than, well, nothing at all which is what we have at the moment. The text on the actual page says literally nothing beyond "help keep Thunderbird alive". The FAQ says "Thunderbird is the leading open source email and productivity app that is free for business and personal use. Your gift helps ensure it stays that way, and supports ongoing development." which tells us almost nothing. And "MZLA Technologies Corporation is a wholly owned for-profit subsidiary of the Mozilla Foundation and the home of Thunderbird." which tells us that donations go to a for-profit subsidiary of the Mozilla Foundation (which I believe is the same entity that owns the Mozilla Corporation, but like most people I am not an expert on this stuff and don't know what that means in practice about how the Mozilla Foundation, the Mozilla Corporation and MZLA Technologies Corporation actually work together).

Maybe donated money will lead to MZLA Technologies Corporation hiring more developers or paying existing developers more? Maybe it'll be used to buy equipment, or licences for patented stuff? Maybe it'll be used to advertise Thunderbird and get it more users? Maybe it'll be used to agitate for the use of open email standards or something like that? Maybe. Maybe some other thing entirely. There's no way to get any inkling.