
Samsung GALAXY Mega 7.0
The latest firmware for Samsung GALAXY Mega 7.0 (SM-T2558) is T2558ZMUANF2 with Jelly(Android Bean), released on June 18, 2014 for 1 region (CSC). Every official firmware build for SM-T2558, 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/T2558ZMUANF2

SM-T255S

SM-T2519

SM-T350X

SM-T357W

SM-T355C
You can find the firmware on the model: SM-T2558 page.
Yes, all firmware files for SM-T2558 are 100% official, original, and obtained directly from Samsung servers.
To flash SM-T2558, 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 SM-T2558.