I like the second one better. Much better contrast and less useless whitespace. I can't see what's wrong with it but I'm also not experienced in UI/UX.
Edit: Also the icons are much better in the second one. The first one has all monochrome icons which makes them much less useful for quickly scanning for the folder that you are looking for.
I think the second one lacks polish in terms of the styling of the UI components. The mockup was really pretty and its use of color and borders/outlines, while the current version seems to have heavier outlines, leading to an overall clunkier appearance. I'll run this by a couple of my UX friends to see if they can express what I'm talking about in more professional terms so I can provide coherent feedback to the Thunderbird devs.
But the things you mention are specifically the things that I think works better in the second one.
I can maybe agree that the first one looks more "polished" but at the same time I actually feel a but uneasy looking at it because my brain has to work harder to discern the UI elements due to the lack of colors and low contrast. Although the lines are heavier in the second one, I find it much easier to filter out the data that I am looking for.
Also the first one wastes screen real estate like crazy. I can only see 7 emails in that one, whereas the second one shows 14. I want all the emails that I receive within a single day to be able fit on a single screen so I don't have to scroll. Frankly, I think the first one sucks.
Fair enough, I suspect that there's no correct answer here short of doing extensive user studies and/or adding a lot of custom theme or UI configuration settings that the team certainly does not have time to work on. still, the UX/UI professionals are professionals for a reason, and it can't hurt to get their opinion specifically.
I appreciate you doing this. I'm keen on the heavier outlines and squashed message list in 115, versus the mockups which I loved. But you feeding that back professionally and constructively, etc, may be productive; it's not healthy for us or the maintainers, for us to be an angry mass of users.
Edit: Also the icons are much better in the second one. The first one has all monochrome icons which makes them much less useful for quickly scanning for the folder that you are looking for.