If you're using a current version of Firefox installed via the firefox_getLatest.sh script, you will need to set up PulseAudio. If you're using firefox-ESR.tcz, then you only need to set up ALSA.
Guides for setting up
ALSA and
PulseAudio are on the Wiki. In practice for ALSA it usually doesn't require much besides loading the right extensions, even though the Wiki makes it look complicated by including lots of extra info. I haven't used PulseAudio myself.
You might also need to load extra A/V library extensions so that Firefox can decode the audio data. Check for errors in printed to a terminal after starting Firefox on the command-line if audio doesn't work in Firefox but does with other programs using PulseAudio or ALSA.