If the list of folders happens to be very long, a search box can be unfolded from the magnifier glass icon in the header of the folder list. The folder tree is reduced to matching folders as you type in the search box.
Right above the search box, a drop-down menu provides ways to reduce the folder list to common scopes (aka “namespaces”) like “Personal”, “Shared” or “Other users”. The selection may vary with the capabilities of your email server.