Monday, February 27, 2017
Beginning March 7, 2017 Google Hangouts will not support phone and/or video calls on Firefox
Google announced February 24, 2017 that when Firefox releases it's newest version (Firefox 52) on March 7, 2017 phone and video calls for Google Hangouts will no longer be supported (though chatting will still be possible). This is due to the fact that in Firefox 52 Mozilla is ending support for browser plugins.
Google Hangouts uses browser plugins in Firefox to enable audio and/or video calls. They are currently working on a solution to allow them to work in Firefox without plugins. Until then they recommend you switch to one of the following supported browsers until a solution is found: