Linux Mint 21.3
Firefox 124.0.1 (64-bit)

When I go to post #2644009, it shows the video player, but:
Almost immediately, the video player gets greyed out and overlaid with the text "No video with supported format and MIME type found."

When I download the video and play it in Celluloid, everything works.

Chromium 123.0.6312.86 plays the video without any problems.

I can confirm the same thing (also 64-bit Firefox 124.0.1 on Arch Linux) but interestingly if I hit the "Download" button or paste the source into the URL bar the video plays in Firefox perfectly fine.

Likewise no problems running in Chromium. Seems to be Firefox's fault to me.

Can confirm it also happens on Firefox, windows 10 here with 124.0.1 firefox as well. I get this error on several videos on this site, not often upon opening the page though; usually I get said error when I pause a video and play it again too quickly or interact with something else on the page, but I can't recreate it consistently.
Normally, this error is thrown due to bad cookies or conflicting extensions, but considering multiple people are getting the error on the same exact post and seemingly no others, something else is probably happening. I opened the page in an incognito window with no extensions and fresh cookies, and it also threw the error. Wonder what's going on here.

WFM on Firefox 115.7.0esr on 64-bit desktop Linux, EXCEPT: there is no audio. The video plays just fine.

If I hit the "Download" button e621 provides, I get a black browser page with the video correctly playing on it, but no audio.

If I download the video with Raccoony (which see), I get a video that plays back just fine in mplayer or VLC, but both ways, no audio.

Running exiftool on the downloaded video yields:

The File entry is redacted, because it exposes the uploader's Windows user name (like, C:\Users\Someuser.)

I just got away from Chrome and now i have to go back because of firefox's mediaplayer?!

surferotter said:
I just got away from Chrome and now i have to go back because of firefox's mediaplayer?!

fwiw this isn't super common, it's been about a month since i saw a vid that gave the ghost midway through and that was only from the forced sample sized version

worst case (besides using a cromium browser) is you can just download the video (heck, sometimes even going fullscreen or going image-in-image can bodge it well enough to view

CODEC support removed? Seems likely.
Sample size works.https://bugzilla.mozilla.org/show_bug.cgi?id=1624103 Looking at Bugzilla shows some people had problems with VP9 decoding. Hmm, I'll try playing with the settings for media.rdd-process.enabled and a few other things.
Haha, setting that to false just makes the tab crash as soon as it tries to play. Damn. Note: For any video I tested.I tried right-clicking on the file and selecting "open with" to check it with IrfanView, and... it hard locks Explorer.exe. This file is somehow malformed.

It's not even a damn WEBM file. It's a MKV file. It needs to get remuxed (not transcoded) and replaced. Hilariously, it seems like the right CODECs so it should be lossless (to remux, not reencode).

> Doc Type                        : matroska
> Codec ID                        : A_VORBIS
Codec: Google/On2's VP9 Video (VP90) Planar 4:2:0 YUV

Possible or direct solution to this issue:

As one of the comments said, in this case Moonlit-Comet, this could be cause because of something conflicting in the browser, i highly recommend disabling ANY extension that interacts with media in anyway, like audio or visual features, i can confirm this, since i had a extension for a audio equalizer (I'm not an expert but i suppose it modifies something in the browser or the media you are trying to play), i disabled and suddenly the error was gone, i recommend trying to disable this type of extensions, and if you find the one that is making this error, i recommend excluding it from e621, anyway, if this work, it was a pleasure to contribute, have a nice day!

Conflicting extension i used:

1."Audio Equalizer" - "Stylish audio equalizer in your browser's toolbar popup." (This is a recommended extension by firefox)

Other useful comments

moonlit-comet said:
Can confirm it also happens on Firefox, windows 10 here with 124.0.1 firefox as well. I get this error on several videos on this site, not often upon opening the page though; usually I get said error when I pause a video and play it again too quickly or interact with something else on the page, but I can't recreate it consistently.
Normally, this error is thrown due to bad cookies or conflicting extensions, but considering multiple people are getting the error on the same exact post and seemingly no others, something else is probably happening. I opened the page in an incognito window with no extensions and fresh cookies, and it also threw the error. Wonder what's going on here.

Little edit:

if the error continues to happen, i could recommend to re-install firefox, or trying to remove data, like cache, cookies, and other things. (do not remove data like passwords and stuff that do not affect websites)

sun-morning said:

Possible or direct solution to this issue:

As one of the comments said, in this case Moonlit-Comet, this could be cause because of something conflicting in the browser, i highly recommend disabling ANY extension that interacts with media in anyway, like audio or visual features, i can confirm this, since i had a extension for a audio equalizer (I'm not an expert but i suppose it modifies something in the browser or the media you are trying to play), i disabled and suddenly the error was gone, i recommend trying to disable this type of extensions, and if you find the one that is making this error, i recommend excluding it from e621, anyway, if this work, it was a pleasure to contribute, have a nice day!

Conflicting extension i used:

1."Audio Equalizer" - "Stylish audio equalizer in your browser's toolbar popup." (This is a recommended extension by firefox)

Other useful comments

Little edit:

if the error continues to happen, i could recommend to re-install firefox, or trying to remove data, like cache, cookies, and other things. (do not remove data like passwords and stuff that do not affect websites)

No, in this case, it was confused because MKV is not quite the same as WEBM. Basically, one is a subset. You have to use a specific WEBM-aimed encoder for your container file to make this work correctly.

Did you confirm that this specific video gets fixed if you do that? It's weird, because troubleshooting step they tried was to disable extensions.

https://github.com/filebrowser/filebrowser/issues/1998 Oh neat, someone else had this issue with MIME types, and only in Firefox. I vaguely remember Chrome and IE doing something different for some logic like this, decades back.