Follow these steps:
Integrating Free Download Manager with Google Chrome:
1-After installing Free Download Manager (FDM) close any browsers opened, open FDM press Ctrl+o or from Options menu in FDM click Settings.
2-On the left side in settings below Downloads select Monitoring on the right pane select or check Google Chrome and click OK to save changes.
Integrating Free Download Manager with Firefox
FDM will pick up downloads from Firefox browser automatically if Firefox was installed after FDM provided "Firefox" has been selected or enabled under Monitoring in FDM settings.
But it is not possible because all users are supposed to using Firefox and other browsers since they installed operating system.So if you installed FDM after Firefox then you need right click on the download link and select "Copy Link Location" and make sure "Clipboard" is selected under Monitoring along with "Firefox" option.Or right click on the link and Download with Free Download Manager in Firefox.
Once you follow above procedure FDM will automatically links copied to clipboard or file will be downloaded with choosing Download with FDM.