Could a firewall be causing the 'HTTP 1.1 403 Forbidden' error in IDM?

Yes, a firewall can definitely be a cause. Your firewall might be blocking IDM's access to the internet or interfering with the download process. Try temporarily disabling your firewall (with caution) to see if the download works. If it does, you'll need to configure your firewall to allow IDM to communicate properly. You might need to add an exception for IDM in your firewall settings.