m.youtube.com - video doesn't play

[webcompat-bot]

URL: https://m.youtube.com/watch?v=mHv_vjvCExw
Browser / Version: Firefox Mobile 54.0
Operating System: Android 7.1.1
Problem type: Video doesn't play

Steps to Reproduce

1. Navigate to: https://m.youtube.com/watch?v=mHv_vjvCExw
2. …

Expected Behavior: to play video in the background (screen off)

Actual Behavior: not playing video in the background (screen off)

From webcompat.com with ❤️

[adamopenweb]

This looks to be a duplicate of https://bugzilla.mozilla.org/show_bug.cgi?id=1355407.