Earlier quoted context omitted.
When it launched in 2013, Hangouts used an NPAPI plugin that worked in most browsers. In 2014, Google ported Hangouts to WebRTC for Chrome only because it relied on non-standard WebRTC features only implemented in Chrome. Note that Firefox did support WebRTC at this time and Mozilla in fact developed its own WebRTC video chat service in 2014 called "Hello" that worked in Firefox and Chrome. In 2015, Mozilla announced…
Thanks for the info. I totally forgot Hangouts was still using an NPAPI plugin, and I didn't realize Firefox dropped support for those too. Doesn't explain why Safari wouldn't be supported though, as Safari still supports NPAPI plugins.
https://support.google.com/hangouts/answer/2944865?visit_id=...