Since Firefox/Gecko 13 jsdouble has been removed, and instead it is possible to use the default C/C++ type double. You should be able to just replace every usage of jsdouble with double in your code.
jsdouble is a C floating-point number type. It is defined to be a 64-bit, IEEE 754 compliant type. See ECMA 262-3 §8.5. Arithmetic on jsdouble values in C should behave exactly like the floating-point arithmetic in JavaScript, except that C and JavaScript may treat not-a-number values differently.
Implementation note: SpiderMonkey has had several bugs involving details of floating-point arithmetic on various platforms. Controlling the behavior of floating-point arithmetic is very architecture- and system-specific, and can be hard to get right. Help in the form of test cases, commentary, and patches is always appreciated.