Firefox Community Forum

Hello All,

I have a somewhat confusing issue that I've only found two posts on Reddit mentioning previously, but there were no responses.

A short summary: On Discord's website, in an audio call, audio will progressively move from stereo/both ears to mono/only the right ear after anywhere from five to sixty seconds. This only occurs on Firefox, it does not occur on Chrome or Edge (with the same extensions, when applicable.) For the record - this also only occurs with the one headset. I've tried with a second headset and don't have the same issue, but I don't believe the Headset is solely to blame since the issue only happens on Firefox.

I have also tested on Firefox Nightly as a fresh install - no extensions, no settings changed, just install and login to Discord.

Specs:

ASUS TUF Dash F15 FX517ZR HP USB-C G5 Essential Dock (Monitors are connected through it) Windows 11 (August update/build 26200.9168) Razer Blackshark V2 Pro RZ04-0453 headset (Connects via USB-A Dongle plugged in to the laptop) Firefox (x64) (154.0.1)

Troubleshooting steps I've taken:

Removing the Audio driver for my computer and re-installing it. Removing/re-installing the drivers for the Headset. Verified all audio related drivers for my device are up to date. Verified the BIOS is up to date (it's solved unusual problems before) Toggling the "media.latency_preference" toggle in about:config (recommendation from a friend) Disabling all extensions on Discord. Disabling Firefox's strict mode on Discord. Verifying all audio related drivers for my device are up to date. Tested on a Firefox Nightly install (the first time it's been installed on my device). A separate browser (Chrome/Edge - the issue is not present) A separate headset (Issue is not present) Downloading Razer Synapse and disabling any "extra" features, and testing with various options on/off. Disabling any and all other Audio devices (speakers and microphones) while using the headset. Charging the Headset to full.

Things I haven't tested, but thought of:

Resetting Firefox in any capacity - I can, but hopefully testing with Nightly is sufficient. A different computer entirely - on my list, when I can next. A new User profile. Testing with no headset at all, and just utilizing the built-in speakers/microphone for the laptop.

Oddities:

Changing audio settings in Windows will "fix" the issue momentarily - for instance the "Spatial sound" option. Changing from one setting to "Off" (or vice versa) fixes the issue for the same ~5-60 seconds until it slowly goes back to the right ear. These options are present for every device, even ones not experiencing this issue.

It's very odd to me because on one hand it may be the headset, but then the issue is only present in Firefox.

Any help or things to test would be greatly appreciated!

Thank you.

(AI written summary of troubleshooting steps taken before making this support thread)

Firefox version: - 152.0.1 Release - 153.0 Nightly

OS: - Windows 11 build 26200

GPU: - NVIDIA GeForce RTX 4070 SUPER

Codec package: - Microsoft HEVC Video Extensions installed from Microsoft Store

Problem

Firefox does not expose HEVC playback support, causing Twitch to disable 1440p streams encoded with HEVC.

The same machine can play the same HEVC Twitch streams in Chrome immediately after installing HEVC Video Extensions.

Expected result

Firefox should report HEVC playback support and allow playback of HEVC streams, as Chrome does on the same system.

Actual result

Firefox reports HEVC as unsupported.

about:support:

codecSupportInfo: H264 SWDEC HWDEC VP9 SWDEC HWDEC VP8 SWDEC HWDEC AV1 SWDEC HWDEC HEVC NONE

Troubleshooting already performed

- Installed Microsoft HEVC Video Extensions - Rebooted Windows - Updated NVIDIA drivers - Verified hardware acceleration is enabled - Tested with a brand new Firefox profile - Tested with Firefox Nightly - Confirmed Chrome can play the same HEVC streams

Additional observations

Chrome: - Twitch 1440p becomes available immediately after installing HEVC Video Extensions - Twitch Stats for Nerds reports:

 hev1.1.6.L150.90.0.0.0.0.0

Firefox: - Twitch reports that the browser does not support the required resolution - 1440p remains greyed out - about:support continues to report HEVC NONE

Reproducible tests

Firefox console:

MediaSource.isTypeSupported('video/mp4; codecs="hev1.1.6.L150.90"')

Result: false

MediaSource.isTypeSupported('video/mp4; codecs="hvc1.1.6.L150.90"')

Result: false

navigator.mediaCapabilities.decodingInfo({

 type: "media-source",
 video: {
   contentType: 'video/mp4; codecs="hev1.1.6.L150.90"',
   width: 2560,
   height: 1440,
   bitrate: 8000000,
   framerate: 60
 }

})

Result: {

 supported: false,
 smooth: false,
 powerEfficient: false

}

For comparison:

MediaSource.isTypeSupported('video/mp4; codecs="av01.0.08M.08"')

Result: true

navigator.mediaCapabilities.decodingInfo({

 type: "file",
 video: {
   contentType: 'video/mp4; codecs="av01.0.08M.08"',
   width: 2560,
   height: 1440,
   bitrate: 8000000,
   framerate: 60
 }

})

Result: {

 supported: true,
 smooth: true,
 powerEfficient: true

}

Additional note

Setting:

dom.media.webcodecs.h265.enabled = true

causes HTML5 codec test (https://html5test.co/) to report H.265 support, but does not change:

- about:support - MediaSource.isTypeSupported() - MediaCapabilities.decodingInfo() - Twitch HEVC playback availability

Question

Is this a known issue with HEVC Media Source Extensions support on Windows 11 build 26200, or is there another requirement for Firefox to expose HEVC playback capability through MediaSource and MediaCapabilities when the Windows HEVC codec is installed and functioning in Chromium-based browsers?

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System Details:

Windows 11

Dual-monitor setup

Browser: Firefox (latest version)

Issue: Fullscreen video shows borders and does not fully cover the screen

Expected Behavior: The fullscreen video should completely fill the selected monitor without showing any part of the Windows desktop.

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