Earlier quoted context omitted.
It works fine in every browser, but you might need to download a small plugin if you're not on Chrome due to WebRTC being a standard but not a standard. Regardless of your browser, it'll heat your system up so you can fry an egg or broil a steak.
Not in FireFox anymore. They sent alerts for a long time saying FireFox was losing support for hangouts, and now it doesn't work at all.
Ironically that's being caused by WebRTC not having compatible enough implementations across all major browsers (and as an aside being disproportionally difficult to debug). Otherwise they would've just dropped the plugin and be done with it much earlier. Though I'm surprised they didn't act earlier.