On the web client (using FF 25.0.1), the first folder in the list does not behave the same as the other folders. I have noticed two things, which may be related.
I am using List View, with the Unread/Newest filter on all my folders.
First, if the first feed in the first folder has unread items, “Mark All As Unread” (from the right-click context menu) does not work on the folder. I have to open the folder and explicitly mark all as read on that feed.
Second, sometimes when I click on the first folder, it behaves as if the All/Newest filter is in effect, even Unread/Newest is shown in the top bar.