The proper way to set Free Download Manager as the default downloader is to disable the other ones. In Firefox, you need to press Ctrl + Shift + A and to uncheck the Disable button next to the plugins. If you're using Chrome, type chrome://extensions/ in the address bar and then uncheck the box next to the Enable option. In Opera, navigate to Tools > Extensions > Manage Extensions and then press the Disable button. If you're running Internet Explorer, press the Alt tab and from the Tools menu select Manage Add-ons. After that, select the plugins from the list and press Disable.