*** YOU NEED TO REGISTER ON THE FORUM BEFORE YOU CAN ACCESS THE DOWNLOAD LINK ***
Once you have registered you will be able to download the firmware by clicking on the orange "DOWNLOAD NOW" button.
INSTALLATION INSTRUCTIONS
http://www.xperiablog.net/2016/03/12/install-xperia-marshmallow-using-flashtool-and-xperifirm-guide/
- Download the FTF firmware file
- Ensure all drivers are correctly installed on your computer as highlighted in Part 1 of the guide linked to above
- Follow Part 3 of the guide to flash the firmware to your Sony Xperia device
XPERIA FIRMWARE DETAILS
This is official stock Sony Xperia firmware.
Model: Sony Xperia Z3+ (E6553)
Firmware version: 32.1.A.1.185_R6C
Region: Russia (Generic) [1295-0592]
Android version: 6.0

Xperia Z3+_E6553_32.1.A.1.185_Russia_Generic 6.0
Sony Xperia Z3+ (E6553) Android 6.0 firmware - Generic Russia