Over on the Fediverse, I had a wish :

My kingdom for a way to tell Firefox to never, ever play audio and/or video for a particular site. In other words, a permanent and persistent mute of that site. AFAIK this is currently impossible.

( For reasons , I cannot set media.autoplay.blocking_policy to 2 generally. I could if Firefox had a 'all subdomains of ...' autoplay permission, but it doesn't, again AFAIK.)

(This is in a Firefox setup that doesn't have uMatrix and that runs JavaScript.)

Sometimes I visit sites in my 'just make things work' Firefox instance that has JavaScript and cookies and so on allowed (and throws everything away when it shuts down), and it turns out that those sites have invented exceedingly clever ways to defeat Firefox's default attempts to let you block autoplaying media (and possibly their approach is clever enough to defeat even the strict 'click to start' setting for media.autoplay.blocking_policy). I'd like to frustrate those sites, especially ones that I keep winding up back on for various reasons, and never hear unexpected noises from Firefox.

(In general I'd probably like to invert my wish, so that Firefox never played audio or video by default and I had to specifically enable it on a site by site basis. But again this would need an 'all subdomains of' option. This version might turn out to be too strict, I'd have to experiment.)

You can mute a tab, but only once it starts playing, and your mute isn't persistent. As far as I know there's no (native) way to get Firefox to start a tab muted, or especially to always start tabs for a site in a muted state, or to disable audio and/or video for a site entirely (the way you can deny permission for camera or microphone access). I'm somewhat surprised that Firefox doesn't have any option for 'this site is obnoxious, put them on permanent mute', because there are such sites out there.

Both uMatrix and apparently NoScript can selectively block media, but I'd have to add either of them to this profile and I broadly want it to be as plain as reasonable. I do have uBlock Origin in this profile (because I have it in everything), but as far as I can tell it doesn't have a specific (and selective) media blocking option, although it's possible you can do clever things with filter rules, especially if you care about one site instead of all sites.

(I also think that Firefox should be able to do this natively, but evidently Firefox disagrees with me.)

PS: If Firefox actually does have an apparently well hidden feature for this, I'd love to know about it.