IIRC, there was s public notice by Firefox about the API change more than a year in advance. Google does not have the resources for implementation? Firefox support is just low prio for Google. I think it's a deliberate (non)-action because instead of switching away from Hangout, people rather start using Chrome... So not-browser-compat seems to help Google :-/ (see also my other comment)
Google has had three years to adapt to other browsers' standard WebRTC stacks, but it was apparently not a priority for the company as long as the legacy Hangouts plugins still worked in other browsers. Mozilla announced in 2015 [3] that it would remove NPAPI plugin support in 2017, so Google had plenty of notice that the Hangouts plugin would stop working in Firefox. Google's new "Hangouts Meet" service is supposed to work with standard WebRTC in Firefox and Edge, but Hangouts Meet is still in beta and its system requirements page still only lists Chrome.
[1] https://plus.google.com/103171586947853434456/posts/39TCW3Pc...
[2] https://webrtchacks.com/hangout-analysis-philipp-hancke/
[3] https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plu...
It's certainly unfortunate that Google hasn't updated their software but Firefox didn't need to remove NPAPI. They chose to do that and when they did they broke software for lots of companies, not just Google. That's a trade off and decision that Firefox made and I'm suffering for it. They had their reasons for making that decision but that decision had costs and downsides for me.