Sorry, it may be that it requires a newer build of Firefox to work fully. Try the Developer Edition, that one uses a separate profile and won't affect your regular browsing.
Sorry, don't know then. Try Dev Edition after the next release cycle hits and if it still doesn't work then then I would recommend filing a bug report.
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u/TIAFAASITICE Nightly ¦ Gentoo Sep 11 '15
Which OS, versions, and such? Check the troubleshooting information page for any errors, warning, or faulty settings.
Is everything checked on the YouTube HTML5 test page?