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(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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Many thanks

Brian

Hi. Recently, not sure of which version as there seems to be a lot of updates :)

I'm currently on 152.0.1 on a Windows 11.

This problem doesn't happen on my Raspberry pi with 152.0 (aarch64) On my Android 13 I have allowed Pop-ups, via about:config, sort of, but there is no option to make Exceptions

So back to Windows 11.

As said I can enter an exception and that works when I load the site I want until I close Fx and I have to do it all again: Enter Domain : Allow : Save etc.

Once, and only once, I closed the [settings] and I'm sure after closing FX and opening it worked fine.

Just closed again and problem has occurred.

The issue arises on my site https://calstock.org.uk which opens a window to a UK weather site, The Met Office. It requires cookies etc. and shows it's own popup.

What I do is open it first in a new window, which is the popup I am referring to, then I load it in an iframe, then I close the popup.

That way the cookie is set by the new window that pops up so there is no query on my main page.

I only have the one extension, uBlock Origin which I have on all devices

Thanks

Hello, Im using localized (Lithuanian) version of Firefox. Several updates back, Firefox started including russian and Polish in preferred languages by default. Personally i know how to remove those for myself. However, since im managing around 300 computers in a school. This starts to become an issue. In Lithuania we only have one official language. And when pages in a school start opening in foreign (russian) language this starts to be quite annoying. I would love to keep using Firefox but i need help.

when i updated linux a newer firefox version was installed but none of my personal settings, passwords or bookmarks installed onto the new firefox version. can't access my previous profil on my computer

Hello,

Since this morning, I haven't been able to access Bsky on my laptop because of a supposed HSTS risk, which means I can't add an exception to visit it. I use Firefox on my Android and going to the mobile version of the website lets me in no problem, so I assume there's an issue with my laptop.

I'd usually wait a few days and let it solve itself but just in case it's something on my end, I'd like to solve it! I'm on 151.0.4 (64-bit), don't use a VPN or an anti-virus, I have a couple of privacy add-ons but they've been here forever and they've never caused a fuss. I can post a screenshot of the error code and the certificate it sends me if it's any help.

I'd appreciate any pointers! Thanks!

I have [[created a backup file|https://support.mozilla.org/en-US/kb/firefox-backup#w_protect-your-sensitive-data-with-a-password]] today with the latest available Firefox and switched to a new OS. Sadly, the backup file somehow seems to have gotten damaged, even if Firefox still recognizes when I'm inputting the wrong password. On Ubuntu, where I originally created the backup and where I tested it again after things weren't working out, Firefox tells me that there is an issue with the file and that I should try a different one (which I don't have). On CachyOS, which I'm currently testing out, restoring the backup never finishes and Firefox doesn't exit cleanly. In hindsight I obviously should 1) have tested the backup immediately, 2) not have wiped my laptop prematurely, and 3) have backed up the folders manually, but I haven't. Now the question: is there some way to extract the account details from the .html file? I don't care much about the rest, but restoring the accounts in some other fashion is gonna be a lot of work.

Hello,

I like how easily I can access bookmarks, tabs from other devices, and the chatbot from the "Tools" bar on the sidebar, but it becomes inaccessible after enabling "Expand sidebar on hover".

See images attached showing before and after enabling "on hover".

Am I missing something? Is there a workaround?

Thank you.

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With the new organization of Settings, my list of cookies has disappeared. I was trying to delete cookies from a specific site, but when I go to that option in "settings," there is no list of cookies.

I have tried restarting both Firefox and my computer and the list is still missing.

Any suggestions on getting it back?

I am trying to get Russian language spellchecked.

I went to this url: https://addons.mozilla.org/en-GB/firefox/addon/russian-ru-language-pack/, installed the pack. Re-opened the browser. But nothing seemed to changed.

I use English locale.

Version 152.0.1 Build ID 20260617213557 Update Channel release User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:152.0) Gecko/20100101 Firefox/152.0 OS Darwin 25.5.0 Darwin Kernel Version 25.5.0: Mon Apr 27 20:38:00 PDT 2026; root:xnu-12377.121.6~2/RELEASE_ARM64_T8103

I just downloaded the latest FF having had an old account. Working fine but I continually get a message that I am on windows 10, which is wrong. I am on Windows 11. The message has worn out its welcome. Is there a way to fix this.