They should have it all working flawlessly at their scale
Insufficient economic incentives.
Firefox is ~4% of the installed base for web browsers (as per externally-visible metrics). The priority for bug fixing is going to look something like Chrome -> Chrome (mobile devices) -> Safari -> Safari (mobile devices) -> Edge -> Firefox.
... additionally, if the same behavior works correctly in Chrome, Safari, and Edge, it unfortunately indicates it's more likely to be a standards-noncompliance bug on Mozilla's side than an error in the YouTube implementation.
Comments
Insufficient economic incentives.
Firefox is ~4% of the installed base for web browsers (as per externally-visible metrics). The priority for bug fixing is going to look something like Chrome -> Chrome (mobile devices) -> Safari -> Safari (mobile devices) -> Edge -> Firefox.
... additionally, if the same behavior works correctly in Chrome, Safari, and Edge, it unfortunately indicates it's more likely to be a standards-noncompliance bug on Mozilla's side than an error in the YouTube implementation.
EDIT: Specifically, it might be this bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1487102