Since Firefox version >=52, the browser has dropped support for ALSA and depends solely on PulseAudio for sound requirements, hence rendering all sites playing audio broken on an ALSA-only system (such as Lubuntu, an official Ubuntu Flavor).
Mozilla has cited ongoing problems and maintenance issues for dropping ALSA support, as explained in the original bug report. This gives the options of either downgrading to an earlier version of Firefox (not really) or switching to a different browser.