The FlashTool application itself does not contain firmware. You will also need (often bundled or linked separately) to download official Sony firmwares. Version 0.9.22.3 works seamlessly with XperiFirm v5.1.1 and up.
Has a steep learning curve and carries highly technical risks. sony flash tool 0.9 22.3 download
: Users on Reddit generally find the tool straightforward, often noting that "it really is that simple" provided the correct drivers are installed. The FlashTool application itself does not contain firmware