Mozilla this week announced that the next upgrade to Firefox, due for release March 19, will automatically block all auto-play audio by default.

"Starting with the release of Firefox 66 for desktop and Firefox for Android, Firefox will block audible audio and video by default," Chris Pearce, a Mozilla software engineer, wrote in a post to the company's Hacks blog.

Only when the user explicitly interacts with the website to launch audio or video content - Pearce cited clicking a "Play" button as an example - will Firefox allow sound to stream.

"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 auto-play and will be blocked if it is potentially audible," Pearce added.

Auto-play blocking will be enabled by default in Firefox 66, currently scheduled for release March 19.

Users may set site-specific controls to allow some destinations to start playing audio as soon as the browser pulls up a page. And auto-play video will be allowed when muted, Pearce said. He recommended that site developers adopt the latter practice, perhaps with an "Unmute" button for users to click. "Note that muted auto-play is also currently allowed by default in all major browsers which block auto-play media," Pearce said.

Google's Chrome has led the pack in stymying audio auto-play, although that browser has taken a looser approach. As long ago as 2013, Chrome began blocking audio that blasted unbidden from an opened tab. Last year, it instituted stricter control over auto-play, though at the same time it declined to block every site's audio.

On personal computers, Google tracks user behavior and "if the user has frequently played media on the site, according to the Media Engagement Index" (MEI), audio will pour from the speakers. The MEI, according to a Google explanatory document, "provide[s] a metric reflecting the engagement of a given user with regards to media playback on a given origin." Sites with high MEI scores - a prime example would be youtube.com - are given a pass on the no-sound rule.

Firefox, by comparison, doesn't deal with nuance.

Chrome also already blocks auto-played Web Audio content, something Pearce said Firefox would only get to sometime later this year. The Web Audio API (application programming interface) is a newer make-sound standard for web applications.