Firefox in WebAssembly

Simon Willison · Simon Willison · 2026-07-16

Puter compiled Firefox/Gecko to WebAssembly using an estimated $25,000 in AI tokens, enabling a full browser to run inside Chrome with all traffic proxied over WebSocket for network access.

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AI-Assisted Coding Culture: Landmark Rewrites, PR Description Backlash, and Prompting Debates

Extraction

Topics: webassemblybrowser-technologyai-assisted-programmingfirefox

Claims

Puter compiled Firefox/Gecko to WebAssembly, making a full browser runnable inside another browser.
The project consumed an estimated $25,000 worth of Claude Opus and Fable tokens, though actual cost was lower due to a Claude Max subscription.
Browser-in-browser traffic must be proxied over WebSocket (Wisp protocol) because sandboxed browser code cannot open arbitrary network connections.
End-to-end encryption is preserved for HTTPS sites traversing the WebSocket proxy, while HTTP traffic is in cleartext.
A parallel WebKit-to-WASM project exists but lacks an accessible public demo.

Key quotes

This is absurdly cool: Puter compiled Firefox to WebAssembly such that the whole browser runs in another browser.

The project used an estimated $25,000 worth of Claude Opus and Fable tokens, but took advantage of a Claude Max subscription plan so cost much less in actual dollars.

I inspected the WebSocket messages and traffic to my own HTTPS site was encrypted whereas requests and responses to http://www.example.com/ were in cleartext.