Firefox Community Forum

(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?

Media resource could not be decoded, error: Error Code: NS_ERROR_DOM_MEDIA_DECODE_ERR (0x806e0004) Details: virtual mozilla::MediaResult mozilla::FFmpegVideoDecoder<62>::DoDecode(mozilla::MediaRawData*, uint8_t*, int, bool*, mozilla::MediaDataDecoder::DecodedData&): avcodec_send_packet error: Unknown error occurred

Browsing this website article I can see that there are embedded Youtube videos in the article but they are not loading, instead I only see blank spaces. I've turned off all extensions, turned off tracking protection, cleared cache and cookies for both the website and Youtube.com, and tried viewing the website in troubleshoot mode.

"I’ve been having trouble with choppy YouTube videos in Firefox. It happens on both Wi-Fi and Ethernet connections. My system is running Windows 10, and I’ve already tried disabling hardware acceleration and clearing my browser cache. The issue persists even when I’m in Safe Mode. I’m using the latest version of Firefox (XX.XX). Any suggestions on how to fix this?"

When I full screen in YouTube, Firefox makes a Fade in animation. Can I disable it and make the default animation works?

Note: I notice that Firefox on Linux doesn't have this feature, I don't know why this feature is only for Windows.

System Information: Surface laptop has Windows 11.

Whenever I play something in the browser while connected to my TV by HDMI, the sound still comes out of the laptop speakers. This happened suddenly, without any discernible reason, and I can't seem to fix it. Everything else works, only content played from Firefox remains in the laptop speakers. I've checked my audio mixer, nothing seems amiss, I've updated Firefox and refreshed the application, still nothing. Hell, I've even tried different cables and HDMI ports, but nothing works. What could be the issue? Why did my audio suddenly stop working, and what can I do about it?

Videos in YouTube play at about 50% brightness in full screen and not full screen, unless hardware acceleration is turned off, my mouse is hovering over an auto play thumbnail, or when the video is not in full screen and I open the right click context menu on the video.

I have an AW3225QF HDR monitor and an RTX 5070ti

I am using FF desktop. All of a sudden, I get no sound through my laptop speakers when trying to watch videos (on YT and FB for example). I still get sound if I use external speakers through the 3.5" jack. I still get sound through other browsers, no matter if it's through the laptop speakers or external ones. So, it is definitely an FF problem. Sound works fine on all other media files on the laptop. Refreshing FF didn't fix it. Nor did reinstalling or restarting the laptop. I tried changing the autoplay settings as suggested in the help article, but still no laptop audio via FF. How can I fix this? Thanks

I've freshly reinstalled Windows 11 and after trying to access and watch a movie on any of these sites I get an error (see the pictures).

Browser version: 137.0.1 (64 bit) Widevine version: 4.10.2891.0

Already tried to disable/enable DRM. Already reinstalled firefox. Already manually installed widevine. No clue what could be causing this issue.

Fire fox is automatically playing videos/sound or both if I click on a tab that has that or also in facebook, etc. How do I stop this? I want to be able to click play when I want to hear/see something.

This is the link to the video that will not play in FireFox. https://www.swingtechnologies.com/UploadVideos/clarkj@pobox.com/S200629_019 B.mp4

Trouble with streaming services(like youtube(only streams ), twitch, music streaming) on nightly Firefox Nightly 145.0a1 (2025-10-11) (x64) Cachy OS Binary from firefox.com