Thunderbird needs INTERNAL REWORK, not UI
Thunderbird does NOT need a fancy and useless "modern" UI. It needs internal rework. Local email storage is SLOW when folders are big. IMAP sometimes hangs on "sending message". Sometimes TB just crashes (rarely, but it happens). On big installations (with lots of accounts and folders and emails) it sometimes says that this or that script no longer responds, even of fast PCs with SSDs. I have some 100 or so installations on win, mac, and linux. And they all have the same issues. So, issues are not OS-related.
Everybody (based on comments here) want a USABLE email client, and TB is committed to produce a "nice" email client. I think we have a problem here.