As detailed in bug 1486665, when ETP strict is enabled, Twitter videos shared on Slack don't play. This bug is about why, when the videos' play buttons are clicked, they say "No video with supported format and MIME type found."

"No video with supported format and MIME type found." - this is bit generic but basically, we tried to stream and decode the video and that process failed. From the original bug, it sounds like we block the video stream via Tracking Protection, which might explain why the media decoder spits this back out? Just a guess, I'll have someone poke a little lower.

Thanks, Jim (and Alastor in advance). I'm hoping we'll be able to have a more descriptive error message :)

Hi, Andrew,
Would you mind to help me capture profiled result with logging? (See this example video)? Or share the slack message link if that is a public channel.
Thanks!

I did a quick check and found the resource is blocked by our security check (NS_ERROR_CONTENT_BLOCKED) per following log. So it seems to me that this issue is more related with the security & privacy. I will move this bug to another component and let them check if this is a bug or an intended behavior. Feel free to move this bug back if you think this is a media issue.

Here is the ways to reproduce the issue,

STR

1. go to about:preferences#privacy and choose Strict mode
2. go to https://mozilla.slack.com/archives/C58D3LAAU/p1677543882974659 (in #media)
3. play that embedded video

Expected.
4. video starts playing

Actual.
4. video can't not start playing and show an error

2023-02-27 23:56:35.419000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 Play() called by JS readyState=0
2023-02-27 23:56:35.419000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 created PlayPromise 1dad1e9d400
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435e800 ChangeDelayLoadStatus(1) doc=0x0
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435e800 Network state changed to LOADING
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435e800 Trying load from <source>=https://video.twimg.com/ext_tw_video/1596895228379774982/pu/vid/1280x720/YJwiqJ5unDGs-zPh.mp4?tag=12 type=video/mp4
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 ChangeDelayLoadStatus(1) doc=0x0
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 Network state changed to LOADING
2023-02-27 23:56:35.448000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 Trying load from <source>=https://video.twimg.com/ext_tw_video/1596895228379774982/pu/vid/1280x720/YJwiqJ5unDGs-zPh.mp4?tag=12 type=video/mp4
2023-02-27 23:56:35.509000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement OnVisibilityChange(): ApproximatelyVisible
[Child 40092, Main Thread] WARNING: Load group requested for media element in inactive document.: file C:/mozilla-source/mozilla-unified/dom/html/HTMLMediaElement.cpp:6606
[Child 40092, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x805E0006 (NS_ERROR_CONTENT_BLOCKED): file C:/mozilla-source/mozilla-unified/dom/security/nsContentSecurityManager.cpp:1541
[Child 40092, Main Thread] WARNING: 'NS_FAILED(rv)', file C:/mozilla-source/mozilla-unified/netwerk/protocol/http/HttpChannelChild.cpp:1986
2023-02-27 23:56:35.529000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435e800 Network state changed to NO_SOURCE
2023-02-27 23:56:35.529000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435e800 ChangeDelayLoadStatus(0) doc=0x1dae20c5400
2023-02-27 23:56:35.538000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 Network state changed to NO_SOURCE
2023-02-27 23:56:35.538000 UTC - [Child 40092: Main Thread]: D/HTMLMediaElement 1dad435ef00 ChangeDelayLoadStatus(0) doc=0x1dad1ee9800