None. A great UX nowadays is open source software running on your own hardware.
For example, you couldn't pay me to use a "webmail" like GMail over my own IMAP server and Thunderbird.
why would a great UX be tied to the source being open or not?
How's that? VLC, GIMP, Ubuntu search and settings. Terrible. Great products, awedul UX.
Wow.. you are the one loving thunderbird. The ridiculous idea of removing menubar and if you enable that - it wastes valuable screenspace.
As somebody who already does this, I wouldn't say the Thunderbird's UX is the real motivation.
I do it for autonomy and avoiding lock-in, but Thunderbird has some frustrating inconsistencies particularly in its mishmash of searching and filtering.