SpiderMonkey is Mozilla’s JavaScript and WebAssembly engine, used by Firefox and by browsers built on its code. It parses web content, executes scripts, and uses just-in-time (JIT) compilation to turn frequently run code into faster machine code. That speed is important for modern web apps, but it also makes the engine one of the most complex and security-sensitive parts of the browser.
In cyber security, SpiderMonkey matters because it processes untrusted code from websites every time a page loads. Vulnerabilities in its parser, garbage collector, or JIT compiler can let a malicious page crash the browser, corrupt memory, or create an entry point for code execution. Attackers often target browser engines because they sit directly on the path between web content and the local system. Defenders respond by patching quickly, restricting script features when needed, and keeping browser releases in sync with upstream engine fixes.