It's not the only codec, but it's the only high quality codec mandated by the spec and supported by all the browsers. G711 is also mandated by the spec but it'

It's not the only codec, but it's the only high quality codec mandated by the spec and supported by all the browsers. G711 is also mandated by the spec but it's a low quality codec intended for speech with a fixed 8kHz sampling rate. There are a few other codecs supported by Chrome and Safari but not Firefox.