Does thunderbird support the file storage, labelling, alias creation, and notes features as well? I thought thunderbird was just for email using IMAP. Does it support JMAP?

And what about them making this has detracted from your experience as a Fastmail user? I've found the desktop site to be perfectly fine, and they recently addressed my primary complain with the mobile app by allowing emails to be saved offline. I haven't found anything worrying that I feel their dev time needs to urgently address that was ignored due to this. Fastmail just works.