>Ah thanks I wasn't able to see the difference between PV_Random() and RandomX().

Random() draws a random number when you call it. PV_Random tells the particle system to draw random numbers (for every single particle) later, when the particles are created.
For example:
var random = RandomX(-10, 10);
for (var i = 0; i < 100; ++i)
    CreateParticle(..., random, ...);

This would create 100 particles with the same random number (for example the same speed). With var random = PV_Random(-10, 10); every particle would have a new random speed.

>Most of the particles have to change their size depending on the current value of GetCon()

True. If it only affects the size, you could set only the size in every timer call again (effect.yellowspark.Size = ...). But you are right: that only makes sense when the fire is finished :)

>On the other hand Fx*Start gets triggered quite often as well when I remember correctly

Fx*Start with temp = false is called exactly once when the effect starts. When effects with priorities are added and removed later, more Fx*Start calls can happen. They will always have temp = true, though.

That's why I wrote if (temp) return; in my example above!