The recent Thunderbird user interface is on the face of it almost indistinguishable from Windows Outlook mail, which is certainly not offering a reason to diverge from the default Windows offering to use an open source mail client.
The following two screenshots are from Outlook and Thunderbird respectively using the same mail accounts, but not same calendar, on the same system. Other than Outlook being "prettier" and having a toolbar under the "menu" there is little practical visual difference.
However, this reddit user will probably not be impressed as Thunderbird is still not beautiful IMO.
I prefer the traditional Thunderbird UI with Vertical View and Table View over the new Card View. I am fine with the new Supernova release because it is still very customizable. I still think Thunderbird has better UI than Outlook or any mainstream web email client. Outlook maybe more "beautiful" by modern standard but it is less useful. I tend to prefer pre-touchscreen era UI.