Well, managed to run XperiFirm.exe by running it in mono.

When I select Xperia X / F5121 it gives a bunch of downloads, but seems there are only latest versions available.
How do I know which android version they are?

I tested by selecting bundled nordic version, and downloaded it; however instead of a single *.ftf file, there are tens of files of type *.sin
Can I perform a flashing with these, or do I need to somehow bundle these to a *ftf first?