The drag and drop not working in firefox is curious, as I used firefox to build PNGEverything. Would you mind telling me what version you use?
I also came across the libpng error and I'm not really sure on how to fix it. I followed the specifications for a PNG file down to a T, and pngcheck (http://www.libpng.org/pub/png/apps/pngcheck.html) gives me an OK for the file.