2.5.1 was the transition period. If you install on modern hardware (BIOS/UEFI), it will likely default to .
use third-party mirror sites (like old-pfsense-downloads.ru or random FTP servers). Those are a great way to install a backdoored router. Pfsense 2.5.1 Download Iso
Once you have successfully installed pfSense 2.5.1, security should be your top priority. Use this quick checklist to harden your new firewall: Pfsense 2.5.1 Download Iso
image before writing it to a USB drive or mounting it in a virtual machine. Verification Pfsense 2.5.1 Download Iso