Allow top-level folders
Currently, ProtonMail sorts all folders under a folder creatively named "Folders". While this looks nice on the web app (and is a much better solution than GMail using labels for everything), in third-party email clients such as Thunderbird, user-created folders all get thrown under Folders. Visually, this is slightly unappealing. Other email servers simply place folders at the top of the file-tree. This is not really important but is a small detail that would be nice to have.