Please go to the site image-12/15 and download the imagefile. After downloading it is recommended to install a VirtualBox service to create a virtual machine from this image. Simply extract the archive and then do a
VBoxManage convertfromraw --format VHD ipfire-3.x-x86_64-20150901.img IPFire-3.vhd
After these steps you are able to create a new VM in VirtualBox with the desired amount of NICs and for the HDD Image select your converted VHD File.
Now you are ready to boot up IPFire-3.x