Up until recently, I was able to launch/open JNLP files in Firefox using Java web start.
Don\'t know what happened all of a sudden JNL
I had the exact same problem. Turned out that the max-heap-size was set to 1024 and missing the unit. The configuration needed to be max-heap-size=1024m.
So apparently invalid memory configuration in the jnlp file will cause this exact behavior.