on my iMac my issue with Safari is that if I tab away from streaming content or such it reduces the quality so when I switch back to that tab or the browser I have to wait a few seconds for the quality to go back up.

this was very noticeable with twitch where I tend to have more than one stream up. I will pop up mail to reply to something and watch Safari drop the resolution behind mail yet firefox does not do this. I haven't found a solution to it yet