In theory, a website that wanted to play video or audio could throw in a '<video controls ...>' or '<audio controls ...>' element in the HTML of the page and be done with it. This would make handling playing media simple and blocking autoplay reliable; you'd ignore the autoplay element and the person using the browser would directly trigger playing media by interacting with things that the browser directly controlled and so the browser could know for sure that a person had directly clicked on them and the media should be played.

As anyone who's seen websites with audio and video on the web knows, in practice almost no one does it this way, with browser controls on the <video> or <audio> element. Instead, everyone displays controls of their own somehow (eg as HTML elements styled through CSS), attaches JavaScript actions to them, and then uses the HTMLMediaElement browser API to trigger playback and various other things. As a result of this use of JavaScript, browsers in general and Firefox in particular no longer have a clear, unambiguous view of your intentions to play media. At best, all they can know is that you interacted with the web page, this interaction triggered some JavaScript, and the JavaScript requested that media play.

(Browsers can know somewhat of how you interacted with a web page, such as whether you clicked or scrolled or typed a key.)

On good, well behaved websites, this interaction is with visually clear controls (such as a visual 'play' button) and the JavaScript that requests media playing is directly attached to those controls. And even on these websites, JavaScript may later legitimately act asynchronously to request more playing of things, or you may interact with media playback in other ways (such as spacebar to pause and then restart media playing). On not so good websites, well, any piece of JavaScript that manages to run can call HTMLMediaElement.play() to try to start playing the media. There are lots of ways to have JavaScript run automatically and so a web page can start trying to play media the moment its JavaScript starts running, and it can keep trying to trigger playback over and over again if it wants to through timers or suchlike.

Since Firefox only blocking the actual autoplay attribute and allowing JavaScript to trigger media playing any time it wants to would be a pretty obviously bad 'Block Autoplay' experience, it must try harder. Firefox's approach is to (also) block use of HTMLMediaElement.play() until you have done some 'user gesture' on the page. As far as I can tell from Firefox's description of this, the list of 'user gestures' is fairly expansive and covers much of how you interact with a page. Certainly, if a website can cause you to click on something, regardless of what it looks like, this counts as a 'user gesture' in Firefox.

(I'm sure that Firefox's selection of things that count as 'user gestures' are drawn from real people on real hardware doing things to deliberately trigger playback, including resuming playback after it's been paused by, for example, tapping spacebar.)

In Firefox, this makes it quite hard to actually stop a bad website from playing media while preserving your ability to interact with the site. Did you scroll the page with the spacebar? I think that counts as a user gesture. Did you use your mouse scroll wheel? Probably a user gesture. Did you click on anything at all, including to dismiss some banner? Definitely a user gesture. As far as I can tell, the only reliable way you can prevent a web page from starting media playback is to immediately close the page. Basically anything you do to use it is dangerous.

Firefox does have a very strict global 'no autoplay' policy that you can turn on through about:config, which they call click-to-play, where Firefox tries to limit HTMLMediaElement.play() to being called as the direct result of a JavaScript event handler. However, their wiki notes that this can break some (legitimate) websites entirely (well, for media playback), and it's a global setting that gets in the way of some things I want; you can't set it only for some sites. And even with click-to-play, if a website can get you to click on something of its choice, it's game over as far as I know; if you have to click or tap a key to dismiss an on-page popup banner, the page can trigger media playing from that event handler.

All of this is why I'd like a per-website "permanent mute" option for Firefox. As far as I know, there's literally no other way in standard Firefox to reliably prevent a potentially bad website (or advertising network that it uses) from playing media on you.

(I suspect that you can defeat a lot of such websites with click-to-play, though.)

PS: Muting a tab in Firefox is different from stopping media playback (or blocking it from starting). All it does is stop Firefox from outputting audio from that tab (to wherever you're having Firefox send audio). Any media will 'play' or continue to play, including videos displaying moving things and being distracting.