We appreciate your report but I was not able to reproduce this issue. I was able to play 4k videos without any issues.

Tested on:
• Browser / Version: Firefox Release 126
• Operating System: Windows 10

Suggestions:
• Clear cache/data/cookies, disable Ad-blocker (if available), or create a new profile (without syncing/importing data) and check again
• If there are any changes made to the default settings of the browser (e.g. in about:config), please revert to the default settings

How to create a new profile:

1. Open a new tab
2. Type in the address bar "about:profiles"
3. Press "Enter"
4. Click on "Create a New Profile"
5. Click "Next"
6. Name your profile
7. Click "Finish"
8. Head to your newly created profile
9. Click "Launch profile in new browser"

Note: You can return to your old profile at any given time by accessing "about:profiles" in the address bar

[qa_22/2024]

i created a new profile and tried your video. somehow this works.
But then i searched for another video, and the video stops playing, skips frames and then starts loading but it does not buffer anything.
See my video (too big to upload on github, so here a wetransfer link).
https://we.tl/t-EYuPYmBqYy

i lowered the quality massively to also upload it here if you don't like to download from wetransfer.

Thanks for the screen rec. I've tried on multiple videos but I was not able to reproduce this issue. Closing this as a duplicate of #132899 and https://bugzilla.mozilla.org/show_bug.cgi?id=1878016

[inv_22/2024]