[Note]: - This bug is macOS High Sierra 10.13 specific [Affected versions]: - ESR 52.3.9, RC 55.0, Beta 56.0b1, Nightly 57.0a1 [Affected platforms]: - macOS High Sierra 10.13 [Steps to reproduce]: 1. Open Firefox. 2. Go to www.youtube.com or other video site that supports selecting video quality under 360p (e.g. www.dailymotion.com) 3. Open a video and select 240p or 144p. [Expected result]: - The video is loaded with the selected video resolution. [Actual result]: - The video does not work and the player displays an error message. [Regression range]: - This is not a regression since it only affects macOS 10.13 Beta [Additional notes]: - Please see the attachments for the youtube error code.

Hello Alexandru - which seed are you testing on? A new seed was just released today.

Hello Marcia, I've tested it on the Beta 4 (17A315i) seed. I used Public Beta 4 for this testing and the one that was released today is the Developer Beta 5. I'll get that one tomorrow and test this again.

Still happens in yesterday's seed: 17A330h

I can confirm Build 17A330h 10.13, and Firefox 55 and Firefox 56 Dev edition cannot play videos from youtube and vimeo

Marcia, would you mind filing a bug with apple?

In the latest seed,17A352a using the latest nightly I cannot reproduce this issue - I played several youtube videos and was able to play them at both 144 and 240.

Hi Alexandru, Can you help check if this issue is still reproducible with Beta56 on build 17A352a?

Just tested again on build 17A360a with Beta 56b12 and the issue is no longer reproducible.

Fixed on Apple's side, resolving this one as WFM to get it off the radar.

Per comment #9, mark 56/57 won't fix.