Get-FileHash .\pfSense-CE-2.5.1-RELEASE-amd64.iso.gz -Algorithm SHA256
If you must use it, the official Netgate archive at atxfiles.netgate.com is your only safe harbor. Keep it behind a double-NAT or isolated from the raw internet if possible. And for goodness' sake, turn off "Allow DNS server list to be overridden by DHCP" unless you want DNS leaks.
Stay safe, and happy firewalling. Did we miss a specific step for writing the memstick image to a USB drive? Let us know in the comments below.