On May 20, the Mozilla SpiderMonkey team announced that starting with Firefox 148, optimizations specifically tailored for asm.js will be disabled by default, with plans to completely remove related code in future releases. Since asm.js is essentially a strict subset of JavaScript, existing websites using asm.js won’t experience any compatibility issues; they simply won’t benefit from the prior performance boosts anymore. Introduced by Mozilla in 2013, asm.js leveraged type annotations to enable JavaScript engines to execute code at near-native speeds, thereby allowing game engines like Unity and Unreal Engine to run high-performance applications in browsers.
The primary reason behind this transition is that WebAssembly (Wasm) has now fully taken over asm.js’s role: it executes faster, produces smaller output files, and offers better cross-engine compatibility. Meanwhile, the optimization pathways developed for asm.js now merely add extra maintenance and security burdens. In their announcement, the SpiderMonkey team acknowledged asm.js’s historical significance as a precursor to WebAssembly and advised developers to recompile existing asm.js projects into WebAssembly to achieve superior performance.