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.
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.
Really? And what about tiny scrollbars?
Oh, how I hate that use of "we"! Please realize: "YOU" does not equal "EVERYONE"
Personally (sample size 1) I use several different PC-like hardware units for productive work, as well as personal use. These units have different hardware/software/OS in general, but some applications are used cross-platform: Email is one. Hence the need for Thunderbird in the first place. I have been using it for no less than a decade and probably much longer - iow as long as I remember.
The fact is that none (0, zero) of the total amount of hardware units I personally use _at_all_ "... have mouse wheels and touch screens" - not a single one.
I am not a robot! I exist, and I'm a core user of that product (which has become increasingly annoying in multiple ways during the years - way too narrow scroll bars are actually a good example of the dev team undermining usability - but there is no feasible alternative).
It seems the team is operating from some assumptions about user preferences that generally does not include (the likes of) me.
(ADDED: which would be fine if my demands were higher than their aility to deliver, but that is not the case. I only ever use the very basic core functions (read, write, folders; not even search) - no UI features added the past 5-10 years have been relevant for me. All of them have cost me time/productivity as anything new in the UI (no matter how minor) requires a breach of habit and a new learning loop, even if just to ignore said new feature.)
So, for the past couple of years my personal use of email has been on a steady decline. I just tend to avoid that kind of activity when/if I can.
ADDED: Don't preach me webmail in replies please. I did write "there is no feasible alternative" because that was exactly what I ment to say. Same goes for "just join the dev team and make a change yourself". I have actually considered these, but then: There IS NO feasible alternative. And that is the singular reason I still have Thunderbird installed even if used sparingly these days