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Are you referring to the macOS client app? It's built in Swift with an FFI bridge to the Rust-powered connectivity layer. Source is here: https://github.com/firezone/firezone/tree/main/swift/apple

I am referring to the web app

I think it's LiveView for the frontend-backend comms and Tailwind CSS for some of the frontend, not sure if that answers your question

-- (non-web) developer at Firezone