
Samsung Galaxy Mega 5.8
The latest firmware for Samsung Galaxy Mega 5.8 (GT-I9158) is I9158ZMUANF3 with Jelly(Android Bean), released on July 16, 2014 for 1 region (CSC). Every official firmware build for GT-I9158, 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/I9158ZMUANF3
The newest build, I9158ZMUANF3, was released on July 16, 2014. No new build has appeared for 149 months, so regular updates for this model have most likely ended.

GT-I9308

GT-I9268

GT-I9228

GT-I9210

GT-I9108
The latest build for GT-I9158, I9158ZMUANF3, was released on July 16, 2014 (12 years ago).
GT-I9158 has had no new firmware for 149 months, so regular updates have most likely ended.
Yes, all firmware files for GT-I9158 are 100% official, original, and obtained directly from Samsung servers.
To flash GT-I9158, 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-I9158.