
Samsung Galaxy S 4 DUOS
The latest firmware for Samsung Galaxy S 4 DUOS (GT-I9508) is I9508ZMUJPA1 with Android 5.0.1 (Lollipop), released on January 6, 2016 for 1 region (CSC). Every official firmware build for GT-I9508, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/I9508ZMUJPA1
GT-I9508 started on Android 4 and has received 1 major Android upgrade; it now runs Android 5.0.1 (Lollipop). The newest build, I9508ZMUJPA1, was released on January 6, 2016. No new build has appeared for 131 months, so regular updates for this model have most likely ended.

GT-I9308

GT-I9268

GT-I9228

GT-I9210

GT-I9108

GT-I9103

GT-I9100T

GT-I9050
The newest firmware for GT-I9508 runs Android 5.0.1 (Lollipop): build I9508ZMUJPA1, released on January 6, 2016.
The latest build for GT-I9508, I9508ZMUJPA1, was released on January 6, 2016 (10 years ago).
GT-I9508 has had no new firmware for 131 months, so regular updates have most likely ended.
Yes, all firmware files for GT-I9508 are 100% official, original, and obtained directly from Samsung servers.
To flash GT-I9508, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing GT-I9508.