Not all codecs were ported at the same time, and then not enabled by default, and when enabled by default it was platform dependent, for others where it worked FF was eating all possible CPU resources and videos were glitching. I remember this as I was using Debian and I was active on /r/firefox, where this [1] link was posted 10 times every day
YouTube turned off flash player as the default in 2015, and VP9 was supported in Firefox at the same time.
YouTube still serves h.264, vp9, and av1.
I was trying to figure out what this could be referring to.