Im trying to write a scritable plugin and I am using mozilla's example below as my guide, as well as looking at firebreath to see how it wraps the code. I am getting stuck on the return value to javascript.
When javascript calls my function the Allocate,HasProperty,HasMethod,Invoke all get called. I return back the result in Invoke and the javascript variable is undefined or crashes the browser when modifying the result.
STRINGZ_TO_NPVARIANT(_strdup("Hello World"), *result);
STRINGZ_TO_NPVARIANT is actually a bit dangerous; when you put a string into an NPVariant object you give ownership of that memory to the browser. However, if you didn't allocate that memory with NPN_MemAlloc things may explode when it tries to release that memory (possibly the source of your crash).
Look at what STRINGZ_TO_NPVARIANT is actually doing and don't use it 'til you understand how it works; until then, you may try performing the steps by hand so you have a better understanding. Allocate memory using NPN_MemAlloc and then strcpy your string to it. I bet this fixes your problem; after you've got it figured out build your own inline functions or whatnot to clean up the code again.
(temp = NPN_MemAlloc(strlen("Hello World")); memcpy(temp, "Hello World", strlen("Hello World")); STRINGZ_TO_NPVARIANT(temp, *result);)