Hello,
I have my new Mixtrack pro FX controller. I started by buying the Pioneer DDJ400 which is almost fully mapped with Mixxx. But I found it too plastic. So I turned it over to take the pro FX. I find that it is of better quality than the DDJ400 … But now there is still the mapping to be done with Mixxx.
Thanks to @ bad1dea5. I tested your mapping. It works for my controller (I’m on Ubuntu).
I am new to controller mapping. I do a tiny bit of javascript so I am limited enough to help with development. I am ready to contribute anyway.
I did again a midi learning of the controller on mixxx just to get the midi codes. From what I see they are not really identical with the Mixtrack pro 3 or the platinum. In addition, it is possible to do a lot of things just with XML mapping without having to go through JS, and I like the default behavior of Mixxx.
The point that seems difficult to manage and which does not yet work with your mapping is the management of pad modes. The pads work very well (for example to map cues points) but without using the mode pads (CUE, LOOP, Fader-cuts, Sampler, FX1, FX2 …). And the best would be if we could also manage the pad modes with shift to increase the possibilities even more. Then add to that the activation of the leds 
I tried to see how to do it and there it is difficult for me to program. One difference that I noticed compared to the mapping of the Pioneer DDJ400 which also has the mode pads, is that the pioneer generates a different midi code when we activate a mode, which really simplifies the task. For the mixtrack FX, the mode does not modify the midi code of the pad.
Do you have any idea how to manage the pad modes?
In any case, thank you again for the work done.