The Fortinet support website provides access to firmware downloads, release notes, and documentation:

The latest verified firmware release for FortiGate is , which focus on stability and security enhancements for enterprise environments. While 7.4.x and 7.2.x are still widely used for their mature stability, 7.6.6 introduces significant refinements for large-scale deployments like the FortiGate 6000 and 7000 series. Review of FortiOS 7.6.6

This version is designed as a high-integrity maintenance release. It does not introduce radical UI changes but focuses on "under-the-hood" performance and resolving vulnerabilities. FortiGate Next-Generation Firewall Cost & Reviews