There's something psychological about the appearance of a low-density display as well. I can't quite explain it, but it causes me a bit of displeasure because it feels wasteful. There are so many pixels available but they aren't doing anything for me. I don't know why this causes me discomfort, but it does.
Finally, it's important for an application like a mail client to not lose sight of the fact that the way people use email varies. I personally use dozens of folders, meaning the density of the folder list should be as compact as reasonably possible so that I don't need to scroll.
But we have much bigger screens than many years ago when thunderbird was initially designed.
Laptop screens stayed mostly the same sizes, so it makes sense to adjust density to the device type you are using or the window size. This is already done for smartphones, mobile websites usually have much smaller paddings. Same is usually done for iOS vs iPad apps.
I find the UX for GMail, for example to be particularly good, not perfect but very very good for the task. The biggest issue I have with gmail is really folder and rule management.