The Internet Download Manager CC extension is no longer officially supported by Mozilla, as of publication, so it's no longer available in the directory of extensions directly downloadable in Firefox 35. However, you can still download the extension from the developer's website. After downloading and installing it, enable it like all other extensions using the Add-on Manager in Firefox. If you have an older version of IDMCC, disable and remove it first from Firefox before installing the newer version from the developer's website.
Click the three-line "Menu" icon in the top-right corner of Firefox and select "Add-ons" to load the Add-ons Manager. Click the "Extensions" tab and locate the old version of IDMCC, if present. Click the "Disable" button next to the IDMCC extension and click "Remove" to uninstall it.
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Visit the official IDMCC website.
Click the green "Install" button located near the top of the page to initiate the installation process. When you click this button, Firefox asks you to verify that you want to permit the extension installation.
Click "Allow" to permit Firefox to download the extension from the IDMCC website.
Press the "Install Now" button to install the extension in Firefox.
Press the "Restart" button to restart Firefox in order to finalize the installation.
Click the three-line "Menu" icon in the top-right corner of Firefox and select "Add-ons" to load the Add-ons Manager again.
Click the "Extensions" tab if it's not already selected to display a list of all extensions currently installed in Firefox.
Click the "Enable" button located next to the IDMCC extension to enable it.