Firefox playing videos takes too much CPU

[Mikhail T.]

Hello! I noticed recently, how much CPU playing a video takes... A 
firefox-process, playing a modest 1080p clip (such as this 
<https://www.youtube.com/watch?v=DrJY_-BIasY>) for example, will show in 
top as taking between 150-160% CPU (and mine are E5-1620 0 @ 3.60GHz), 
with Xorg taking /another/ 30%. If I pause the video, CPU-usage drops 
down to single-digits, so it definitely is the playback...

If I download that same video (using youtube-dl), and give it to 
mplayer, the playback will only cost about 60% CPU for the mplayer 
itself -- plus 20% for the Xorg. Mplayer using the simple "x11" 
video-output, nothing particularly fancy. The video card is an old 
Radeon (5670/5690/5730).

Is there a good explanation for this discrepancy? Thank you! Yours,

    -mi