The following table shows which Fire TV device runs which Fire OS version.

Fire TV Device                            Fire OS Version

Fire TV Stick 4K (2018) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Edition – Insignia HD (2018) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Edition – Insignia 4K (2018) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Edition – Toshiba HD (2018) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Cube Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Edition – Toshiba 4K (2018) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV (Gen 3) Fire OS 6 — Based on Android 7.1 (API level 25)
Fire TV Edition – Element 4K (2017) Fire OS 5 — Based on Android 5.1 (API Level 22)
Fire TV Stick Basic Edition Fire OS 5 — Based on Android 5.1 (API Level 22)
Fire TV (Gen 2) Fire OS 5 — Based on Android 5.1 (API Level 22)
Fire TV Stick (Gen 2) Fire OS 5 — Based on Android 5.1 (API Level 22)
Fire TV Stick (Gen 1) Fire OS 5 — Based on Android 5.1 (API Level 22)
Fire TV (Gen 1) Fire OS 5 — Based on Android 5.1 (API Level 22)

Source: https://developer.amazon.com/docs/fire-tv/fire-os-overview.html