Firefox 148 Disables Asm.js as Mozilla Sunsets OdinMonkey

Mozilla is retiring asm.js, the statically-typed JavaScript subset it introduced in Firefox 22 back in 2013 to run C/C++ codebases at near-native speed on the web. As of Firefox 148, SpiderMonkey disables asm.js optimizations by default, and the code will be removed entirely in a future release. Existing sites will continue to function because asm.js is still valid JavaScript and will run through the standard JIT, though performance will revert to normal script speeds.

The reasoning is straightforward: WebAssembly has effectively replaced asm.js, and most workloads have migrated. Maintaining a parallel pipeline adds engineering overhead and broadens the VM’s attack surface for diminishing benefit. Mozilla is urging remaining asm.js publishers to recompile to WebAssembly, which now offers a more advanced toolchain, faster execution, and smaller binaries.

The move closes a notable chapter in browser performance history. Asm.js proved that web technologies alone could deliver native-class speed, paving the way for WebAssembly’s arrival in Firefox 52. In SpiderMonkey’s Norse-themed lineage, the OdinMonkey compiler gives way to BaldrMonkey and RabaldrMonkey, the optimizing and baseline WebAssembly compilers that now carry the work forward.