In 2015, Mozilla announced that it would drop support for NPAPI plugins in 2017, giving websites using NPAPI plugins ~18 months to switch to a non-plugin solution. When the NPAPI deadline came, Google announced the Hangouts would stop working in Firefox, they were "actively working to develop a solution" for Firefox, and until then Hangouts users should use another browser:
It's 2018 and Google is still working to support WebRTC in Firefox, but they're definitely working on it. In fact, WebRTC Hangouts does mostly work in Firefox now, but only for users with a G Suites enterprise account whose admin has manually enabled Hangouts. I don't know why.
"Hangouts Meet" is a different product using WebRTC whose support for Firefox is still in development.
I've been documenting this saga on this Mozilla wiki as part of the NPAPI deprecation and Win64 Firefox rollout: