This thing is weird as hell.
I just imported my large webssite and now i want to create thumbnails. Works fine. But it is not setting them as featured. So i go and try to reset manually and then update the posts. And weird thing is that on my old computer with Linux and Firefox is setting featured images but on new windows 7 and latest Firefox is not setting them.
What my browser and system have to do with setting a featured image on wordpress that is online?