Firstly, make sure you have the latest version of the application installed on your PC. After you install the latest version, restart Mozilla Firefox, check the Allow this installation box and click Continue:
Once the plugin is installed, restart again the browser, and download a file from Internet. Free Download Manager will launch automatically and you will be able to select a destination folder, a group (music, software, video) or to schedule the download:
After that, press OK and wait for Free Download Manager to complete the downloading process.