What is the Mozilla Firefox's Download Manager?

From Mozilla Firefox Help

The Download Manager is a feature of Firefox that allows you to
track and manage your past and current file downloads. It is a convenient way
to escape the clutter of multiple downloading windows while still preserving
access to download statistics.

The Download Manager will appear when you download files, when you select
Tools > Downloads, and when you press
CtrlCmd+JCtrlCmd+Y.