They're cheating on the screenshots in the survey. The "current thunderbird" screenshot is taken at half the resolution (or twice the font size) of the "thunderbird redesign" screenshot, giving their redesign twice as much space with which to display the same information. Obviously it's going to look better with that advantage. I'm not sure exactly what it'd mean to "trust" a user interface, but I'm definitely sure I wouldn't trust any of that survey's data.
That's not the only trick they use. Note the choice of plaintext vs. HTML mail (even though current TB supports HTML just fine), interesting looking mail vs. mock-ups of bad spam, different screen aspect ratio, and of course the leading survey questions.
Note how the "tasks" list is cut off because it simply no longer fits the screen (where the current version has enough space to show it), missing UI elements in the new design (the entire quick filter bar, which would reduce the available space to 3 (!) e-mail headers), the harshly truncated and thus useless subject lines in the new design, the lack of the "this is a draft" toolbar in the new design, that was added to make the old one look more crowded.
Thunderbird is forgetting their target audience. I suspect that people who like shiny over features have long switched to web interfaces. The new design would take away what used to appeal to their remaining users.
