[Nawid3333]

The sound can now play on another device than the system's default, headphones for example. The audio button's list gets an Audio output part with three rows: Default output, the device chosen, and Choose an output…, which opens Firefox's own device chooser.

How it routes the sound

• All of the player's sound runs through Web Audio, and Firefox has no AudioContext.setSinkId.
• For a chosen device, the graph's end goes into a MediaStreamAudioDestinationNode, and a hidden <audio> plays that stream on the device (setSinkId).
• The graph moves over only once that element plays, so the sound never plays on both outputs or on neither.
• For the default output nothing changes: anyone who never chooses gets exactly the old path.

Measured in Firefox 158 first (extension pages; the harness answers the chooser with media.navigator.permission.disabled):

Question	Result
selectAudioOutput, element setSinkId	yes
AudioContext.setSinkId	no
In a page's iframe without allow="speaker-selection"	refused (NotAllowedError, permissions policy); content.js now gives the player's iframe the token
selectAudioOutput without a click, even with a known deviceId	InvalidStateError: requires transient user activation
The saved device in the next video's player (a new document)	NotFoundError

So each new video starts on the default output and says so

• The status line shows "The sound plays on the default output: click here to play it on Headphones."
• One click opens the chooser with that device already picked, or with nothing picked if the device is gone.
• A device unplugged while it plays (devicechange) sends the sound back to the default output, with the same notice.
• Choosing from the list clears the notice.
• The status line gained an optional click handler for this. Messages without one behave as before.

Not established

• Whether Firefox's real prompt can be remembered, so the chooser no longer appears.
• The latency the detour adds, which affects A/V sync.
• Whether more than 2 channels get through the detour. The detour follows the video's channel count either way.

glm-5.3-flash as helper and checker

• It drafted the unit tests and the menu part. I checked both and adapted them.
• Its two reviews found 6 real bugs, each now with a test that is red without its fix:
  • the kept choice was read once, so "Default output" was undone by the next video;
  • a route overtaken by a newer action could report a false failure or touch a closed context;
  • choose() could save a choice the user had already abandoned;
  • the channel count did not follow the video;
  • a click for a device that is gone did nothing;
  • a notice went stale after a choice from the list.
• One finding was wrong and left out: new Audio() does not set autoplay.
• Its design check confirmed that no Firefox API avoids the click per new document. Only a player document that lasts across episodes would, which would be a much bigger change.

Tests

• AudioOutputDevice.test.mjs (20).
• statusLevels.test.mjs: a clickable message; both tests are red on the old StatusManager.
• audio-output.e2e.mjs in a page's iframe: choose a device, the next video's notice and its click, back to the default. It passed in Firefox 158 and skips on a machine without audio output (CI's runners may have none).
• Lint, typecheck and unit tests (3158) green locally.
• 7 strings in 16 locales (gemma4:cloud, checked; six "choose it again" phrasings fixed by hand).

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