https://www.bleepingcomputer.com/news/software/after-chrome-and-edge-firefox-also-gets-option-to-mute-sites-with-autoplay-sound/

Mozilla has added an experimental feature to the Firefox Nightly edition browser (63.0a1) that lets users control if a site is allowed to autoplay sound when it loads.

The new feature works by showing a popup notification from the left side of the URL address bar, similar to the classic Firefox push notifications that we have all learned to hate.