Google Hangouts Temporarily won’t support phone and video calls on Mozilla Firefox
If you use the Firefox browser to access and use Google Hangouts, please be aware that there is a temporary issue causing phone and video calls to not work in the Firefox browser.
The work-around is to use another browser. Hangouts will continue to work in Google Chrome, Safari and Internet Explorer.
Please see this blog post for the full details: https://gsuiteupdates.googleblog.com/2017/02/google-hangouts-temporary-issues-with-firefox.html