This kind of extension is used when your device's output sucks and "cranking it to 140%" is the only way to make it audible (usually mangling the quality). Possibly used by hard-of-hearing users too. Or maybe they're stuck on monitor speakers and "just pay $15 for cheapo cans" isn't an option, and the monitor's OSD controls are clunky and awful (they always are).

I've used "volume booster" features in apps before when listening to badly mixed podcasts/audiobooks, because turning up the system volume on a phone makes other apps too loud. I could see a desktop situation that mirrors this for someone, a setting in FF is faster/easier than the windows volume mixer.