FF: No video with supported format and MIME type found

[sourcejedi]

Steps to reproduce

Our unencrypted group chat has various videos we uploaded from our phone apps (Android / iOS).

Outcome

Now, it shows preview image, but it says No video with supported format and MIME type found as soon as you press the play button.

If I re-upload the video in a new encrypted room, I can play it. If I re-upload the video in a new unencrypted room, I can't.

"not a browser-wide issue, if I download a video, then open it up through a file:// URL in FF, it plays just fine."

When downloaded, the videos show as .mp4 files, VLC reports they are h264 video and AAC sound.

According to a similar report on the support channel also: "discord's embedded player works fine as well", and "this also worked just fine up until... a month or two ago I think?" https://matrix.to/#/!IemiTbwVankHTFiEoh:matrix.org/$nMhDbET-bc24h33nrbLc7Mp229KNgZk1LJ6ifWvIr8Y?via=matrix.org&via=envs.net&via=mozilla.org

We are sure we used to be able to play the videos as expected, in Element Web on Ubuntu 22.04. Possibly as recently as 2024-10-14, our second-last video upload. But we can no longer play the videos when using our default browser (Firefox). We have tried a Ubuntu 24.04 computer, a Ubuntu 22.04 computer, and a Fedora Workstation 40 VM.

We can still play the videos using Chrome, Element Desktop, or Element Android.

Operating system

Ubuntu 24.04 / Ubuntu 22.04 / Fedora Workstation 40

Browser information

Firefox 132.0 / 131.0.3

URL for webapp

app.element.io

Application version

Element version: 1.11.83 Crypto version: Rust SDK 0.7.2 (7a21514), Vodozemac 0.7.0

Homeserver

matrix.org

Will you send logs?

Yes

[spookyvision]

this happens to me too on MacOS with FF 132.0. Can also confirm it used to work, but am unable to provide details as to when.

[ApostolosB]

Are you sure this is an element issue and not a firefox issue. Haven't tried on element but i seem to get it on occasion on other videos on the web.

[sourcejedi]

After further testing: I have an issue with unencrypted rooms only. I have edited the description.

I can explicitly say that I have not found the root cause. I hope that an Element dev will be able to look at this, now reproducible defect, and find the cause. I have submitted logs by running the /rageshake command in Element Web, and filling in this issue URL.

If you have suggestions for tests, logs, etc that would help find narrow down the cause, including from the Firefox end, I'm all ears :-). AFAICT these are standard phone videos, a format which Firefox is normally happy to play, even using the OpenH264 decoder that Firefox can download from Cisco on-demand.

[neobrain]

Also observing this on Firefox+macOS. Downloading the video and opening it in the same browser works fine.

Upon trying to play the video in Element, the browser console says this:

The resource at “https://matrix-client.matrix.org/_matrix/media/v3/download/matrix.org/rcxtqIuHBGBXtVpHJGIiftcv?allow_redirect=true” was blocked by OpaqueResponseBlocking. Reason: “after sniff: media request”.

I tried flipping browser.opaqueResponseBlocking in about:config to false, which didn't fix the bug but merely changed the log error to another:

HTTP load failed with status 404. Load of media resource https://matrix-client.matrix.org/_matrix/media/v3/download/matrix.org/rcxtqIuHBGBXtVpHJGIiftcv?allow_redirect=true failed.

[t3chguy]

Looks like this is an upstream bug with Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1921989