Hangouts on Chrome uses a variety of Chrome-specific APIs to make video and audio calls work (or at least it used to). For other browsers, Google developed a web plugin, but that became a problem when Firefox dropped support for plugins (besides Flash) back in March. It has taken a long time, but Hangouts finally works in Firefox again.
At least on Firefox, Hangouts now uses WebRTC for video and audio calls, meaning no plugins are required. In another tweet, Google Product Manager Serge Lachapelle said the feature is only supported on consumer Hangouts at the moment, but it is coming to Hangouts Meet soon.[“Source-androidpolice”]