Starting with the release of Firefox 66 for desktop and Firefox for Android, Firefox will block audible audio and video by default. We only allow a site to play audio or video aloud via the HTMLMediaElement API once a web page has had user interaction to initiate the audio, such as the user clicking on a “play” button.Users will also be able to whitelist sites, for example if you want YouTube or other sites to continue to autoplay sound, you will be able to configure these permissions via the site information panel.
Any playback that happens before the user has interacted with a page via a mouse click, printable key press, or touch event, is deemed to be autoplay and will be blocked if it is potentially audible.
Firefox 66 is scheduled to be released on March 19, 2019. Overall, this is a great move as it's very annoying to open a couple of links and suddenly hear music or someone speaking.