Recently I tested all audio addons, which were updated during the last 12 months. Volume Booster 600% by 0xPats is the only one, that amplifies Bandcamp. And it works relyably on most websites, even on unstable live streams. Its secret is the severe webRequest permission – usually a risk, but not in this case: the first version (1.2.0) is perfectly clean, updates won‘t be necessary for a long time. Long story short: For now, this is Firefox‘ best audio addon!