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I used it last night and it worked for me. I accidentally forgot to switch to Chrome to use Hangouts and was pleasantly surprised Firefox57 just worked.

Maybe you have an add-on that disables WebRTC?

Same here; tested on FirefoxDeveloperEdition for Mac. Maybe this is a slow roll-out on the Google side.

Slow roll-out is so much more likely.

WebRTC has been supported for a while in Firefox, and no nightly will make Google serve you with a proper version. It's entirely up to Google.

Works for me on Firefox 57. And not only does it work, the echo cancellation works quite a bit better.