After scanning over 15 pages of XDA discussions (specifically the Xperia Z, Z1, and Z3 Compact forums), a clear consensus emerges:
Flashtool-0.9.13.0 is a Windows-based (and Linux-compatible via wrapper) utility designed to flash firmware files onto Sony Ericsson and Sony Xperia smartphones and tablets. Unlike its successor, the New Flasher (part of XperiFirm integrations), version 0.9.13.0 uses a robust, time-tested engine that handles .ftf bundle files with exceptional reliability. flashtool-0.9.13.0