The audio autoplay ban will apply to both HTML5 audio and video elements used for media playback in modern browsers, meaning Firefox will block sound coming from both ads and video players, the most common sources of such abuse.
Mozilla's move comes almost a year after Chrome took a similar decision to block all auto-playing sound by default with the release of Chrome 66 in April 2018. Chrome hit a few snags with older Flash-based games, but after a few modifications to its sound blocking procedure, the audio autoplay ban policy returned in Chrome last fall.
Mozilla similarly announced its plans to block auto-playing sound last July, and initially said the feature would launch in Firefox 63, before being delayed to v66, scheduled for release next month.
In a blog post today, Firefox engineers are officially warning website developers to adapt their sites and apps to take into account the new ban.
On the user interface side, when Firefox 66 will access a web page with auto-playing sound --either from an audio or video element-- they will see a new icon in the URL bar that tells them that sound has been blocked on the page.
Users can click this button and allow the page to play sound if they wish to.
Sites can be added to the sound autoplay block list at any point by clicking the "information" icon in the site's URL bar, or by visiting the Firefox about:preferences#privacy section and editing the exceptions list. This option also lets users disable the sound autoplay policy altogether if they don't mind auto-playing ads and videos.
Besides Chrome and Firefox, Microsoft similarly announced plans to block auto-playing sounds in Edge, but the feature never made it to production.