Starting with version 66 of the Firefox web browser, a new feature has been introduced that can block all the automatically playing media. This feature is useful if you feel annoyed when you visit a website and suddenly it starts to play music in the background or it starts to play news videos embedded somewhere in the web page. Most annoying websites are those that keep playing one news video after another non-stop. I don’t know if anyone pays attention to these videos as they become nuisance after some time.
By default, Firefox blocks the auto-playback of all type of media on all websites. When you open a website with auto-playing video, Firefox prevents the auto-play and displays an icon in the address bar to this effect. You can click on this icon and Firefox will give you options to allow or block the auto-playback. But this is site specific setting and applies to the current site only.
Suppose, you don’t mind auto-play but you do not want to have websites auto-play media when the tab is not in focus. For example, you opened a video and then switched to some other tab. By the time video loads, you have already switched to a different tab. Now if you do not want the video to auto-play in the background tab, then you can change a setting in Firefox.
For this you have to type about:config in the address bar and press Enter. After this you can look for block-autoplay and toggle the value of a setting named media.block-autoplay-until-in-foreground and change it to false.
If you want to disable this feature completely, then you can do so from Firefox preferences. Just type about:preferences#privacy in the address bar and press Enter. Scroll down a little and uncheck the option Block websites from automatically playing sound. This setting applies on all and any websites that you access through Firefox browser.