After Firefox dropped support for plugins (aside from Flash), making video and audio calls to work using Google Hangouts wasn’t possible on the browser—until today, that is. Hangouts on Firefox now works thanks to the use of WebRTC. This means the app works on the browser without the use of plugins. One caveat though is it only works for the consumer version of Hangouts at the moment, support for Hangouts Meet is coming at a later date.
Source: Android Police