SpiderMonkey 45 is not binary-compatible with previous releases, nor is it source-code compatible. It's the same language, just not as fast. On remaining platforms the JIT is disabled JavaScript code runs in an interpreter, as it has in previous versions.

On several other platforms (SPARC, MIPS), the JIT is provided but not supported. It is supported on x86, x86_64, and ARM architectures. SpiderMonkey 45 includes a just-in-time compiler (JIT) that compiles JavaScript to machine code, for a significant speed increase. If you are compiling with Microsoft's Visual Studio, note the minimum supported version is MSVC 2013. Compiling it requires a C++ compiler, and the JSAPI can only be used from C++ code. SpiderMonkey 45 is supported on all platforms able to run Firefox 45. You may wish to make the bugs block bug 837921 (aliased as 'sm.embedding'). Or file bugs at under Product: Core, Component: JavaScript engine.

Please let us know about your experiences with this release by posting in the -engine newsgroup. It also contains new language and API features described in detail below. It continues to improve performance over previous SpiderMonkey releases. SpiderMonkey 45 is the JavaScript engine that shipped in Firefox 45.

Firefox 45.0 2