Saying goodbye to asm.js
Mozilla's SpiderMonkey team announced the removal of asm.js support from Firefox, starting with version 140 (scheduled for release in late 2026). Asm.js, introduced in 2013, allowed C/C++ code compiled via Emscripten to run at near-native speeds in the browser by using a subset of JavaScript. However, the emergence of WebAssembly (Wasm) in 2017 provided a more efficient and standardized alternative. The deprecation process began in Firefox 135 with a warning console message, and asm.js will be fully removed in Firefox 140. Developers who still rely on asm.js must migrate their code to WebAssembly, which is supported by all major browsers. The move simplifies the SpiderMonkey engine, reducing maintenance burden and improving performance for other features. The announcement emphasizes that asm.js was a critical stepping stone but is now obsolete.
Developers must migrate asm.js code to WebAssembly before Firefox 140 ships.