In Advanced > Menu Configuration > Mail Commands, when  the options "Mark
Folder Read" is active, then this menu item is always shown on the top of the
QF tab popup list. This should be modified to only show up if the folder has
unread items in it.

At the moment, the menus are pre-created and not rebuilt every time a
QuickFolders tab is clicked. However, since folders with unread email get the
class "has-unread" applied to them a simple workaround is hiding the command
depending on the owning button property:

toolbarbutton:not(.has-unread) #folderPaneContext-markMailFolderAllRead {
        display: none;
}

This is not 100% perfect as folders which do have subfolder with unread mail
are classified in the same way, but it's a good first approximation.

Implemented in 4.11, released on 16/10/2018