Like many Internet users, you probably regularly open several pages at the same time, in different tabs, to read them later for example. Only here it happens, it sometimes happens that some of these tabs are not very cooperative with the notion of “stop blowing myself up, I'm reading”: some unscrupulous web pages do not hesitate to launch videos or music in the background.
Sound that is therefore played against your will. Worse yet, sound that's playing in a tab you're not even on and may have a hard time finding! It's irritating, but Mozilla should help you find the offending tabs soon.
In reality, the thing is not new: Google had already proposed a similar solution in Chrome. Users of the latter can indeed know which tab is emitting sound, and thus close it if necessary.
By activating a hidden option, it is even possible to mute the tab without actually closing it.
In a future version which is not yet specified, Firefox will therefore integrate a functionality of the same barrel, with one more: the function allowing to mute the concerned tab should not be hidden, which is rather good news.
In other words, in the near future we will be able, whether in Chrome or Firefox, to open several tabs at the same time without fear that one of them emits unwanted sound which would be difficult to find. Now we have to wait for all browser editors to do the same, and maybe one day the authors of these pages will realize that they are not very smart. But let’s not dream too much of a utopia that is unlikely to happen.