Does anyone know, how i can get rid of these Firefly artifacts? I don’t use any glass material at all, and I’m not using a sun lamp either… even the mesh lamp (emitting plane) is causing these fireflys. Is it because I’m using glossy-mix shader? 
It could be because you have a light inside a closed mesh.
Try playing around with the clamp(it will help a little)
I’ll try… 
but there just one emission mesh outside
UPDATE: Thanks a lot ! That’s exactly what I needed 
But anyway, will there be some improvements in the future?
In my experience the clamp feature works very well in Cycles.
Fine-tune it raising the number until you get to the point where fireflies are mostly gone (sampling will do the rest), but you don’t loose reflections etc.
If you use it, you can load your render into GIMP, and use the Despeckle filter. (Filter > Enhance > Despeckle). That’s what I did and it took care of the problem. Hope this helped!
Is the caustics on or off?