1] Open your Firefox browser and download idmmzcc.xpi file.
2] Now open this file using your Firefox browser.
Note: If Firefox is not listed in open with box, click Browse button and go to C:\Program files\Mozilla firefox and select Firefox.exe file.
3] Now the Install Add-ons popup dialogue box appears within few seconds and click Ok to install the IDM CC Add-on plugin.
4] After the installation is complete, Restart the Firefox.