Your v21 private build crashes my Nvidia display driver (BSOD) on this youtube video: http://www.youtube.com/watch?v=3KMW-a0Gfo4
(it's the trailer for the new Plants vs Zombies 2). This is the first time I came across this and is 100% reproducible. Other youtube videos played just fine (at least, for those I have interest in viewing).
Using SRWare Iron (backup browser based on Chrome) in the same machine plays the video just fine. I haven't tried using a vanilla Firefox 21 to compare. So, I'm not sure if this is a problem with your build or with Firefox v21 in general.
My machine is a Dell Latitude D630 (Core2Duo T9300@2.5GHz, 3.5GB RAM) with WinXP Pro SP3.
Video is Nvidia Quadro NVS 135M.