It's because mac doesn't support vp8 hardware decoding.
use this extension to force youtube to use h264 https://addons.mozilla.org/en-US/firefox/addon/h264ify/
Blame apple
Blame apple
and Google, for not doing the minimum thing they could have done and served you the best encoding for the platform.
It is almost if the companies you actually give you money to (or eyeballs) do not care at all for your interests.