The absolute final firmware version for the FortiGate 80C is FortiOS 5.6.12
If you absolutely must run an 80C, update it to 5.2.13, disable the SSL VPN, turn off deep inspection, and plan your hardware refresh for next week —not next year. fortigate 80c latest firmware version
If your organization has held an uninterrupted FortiCare contract that includes legacy hardware (rare today), you can find the firmware under: Support Portal → Firmware Images → Filter by Product (FortiGate 80C) → Select Branch 5.2 The absolute final firmware version for the FortiGate
To understand why the latest version is 5.2.13, let’s look at the progression: update it to 5.2.13
Since January 2023, Fortinet no longer provides technical support, hardware repairs, or security patches