sudo nano /usr/lib/nspluginwrapper/i386/linux/npviewer
Add the following line right before the LAST line of the file.
export GDK_NATIVE_WINDOWS=1
The final look would be :
#!/bin/sh
TARGET_OS=linux
TARGET_ARCH=i386
export GDK_NATIVE_WINDOWS=1
. /usr/lib/nspluginwrapper/noarch/npviewer
That's all! See you.