Puter Compiles Firefox to WebAssembly — a Full 233MB Gecko Browser Running Inside Another Browser

Simon Willison / Puter·medium signal

Puter compiled Firefox/Gecko (chosen for its strong single-process support) to WebAssembly, shipping a 233MB gecko.wasm plus 18MB of chrome assets so an entire browser runs inside a browser tab. Network traffic tunnels through Puter's servers over the Wisp protocol via WebSocket (HTTPS stays end-to-end encrypted; HTTP is cleartext), and the build reportedly consumed ~$25K of Claude Opus/Fable tokens — dramatically cheaper under a Max subscription. It's a striking proof of how far agentic coding plus WASM can push the browser sandbox, though server load during the HN spike exposed the resource ceiling.