This is an experimental Firefox 57 download manager with multithreading
support.

It can download files with multiple connections to the server. Depending on
the network condition, this may increase the download speed.
Click the toolbar button to open the download panel. Alternatively, right
click the button to open in a new tab or window. Download tasks can be created
by clicking the buttons at the bottom of the panel, or in the context menu of
links on websites.
Monitoring download can be enabled in the options. When it is enabled, files
larger than a configured size will automatically trigger the download dialog.
Click "continue in browser" to switch to Firefox's built-in download.