I have to disagree. I've been using Thunderbird for well over a decade and I appreciate that the UI hasn't changed much. As I get older I much prefer software that changes slowly and incrementally, if at all - and sometimes no change is good, especially in the UI.

UI is the part of software most prone to fashion. Fashions go in and out, and UIs and their usability goes in and out with them. I would rather stick to a usable, well-known UI than have TB be trendy and strictly worse, like the "flat everything" fad, or the "you don't need options" fad, or the "header bar" fad.