Disclaimer: This post is for informational purposes. Always verify compatibility and security requirements before flashing firmware on network hardware.

: FortiOS 6.0.18 (or the latest patch in the 6.0 branch).

While the Web Interface (GUI) is easier, using a TFTP server to upload the firmware via the Command Line Interface (CLI) is much more stable for older devices.

You cannot jump from firmware v5.2 directly to v6.0. You must follow the "upgrade path."

FortiGate 60D Firmware Download: Definitive Lifecycle, Upgrade, & Installation Guide