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> All the latest developer tools in beta, plus experimental features like the Multi-line Console Editor and WebSocket Inspector. (from https://www.mozilla.org/en-US/firefox/developer/)

This is the biggest difference: Beta dev tool and experimental features.

That must be pretty outdated, because I'm on Firefox ESR and I have used the multi-line console editor and WebSocket inspector.

The multi-line console editor is already on stable for me, at least. It's quite good; definitely recommend trying it out.