It's a well known fact that the the built-in hardware acceleration of Flash Player does not play well with "older" hardware. Mac OS X supports hardware acceleration only on quite recent GPUs and eg. mine is not one of those (my MacBook Pro is from 2006 December). AFAIK Chrome solves this by detecting older GPUs and "manually" disabling hardware acceleration in Flash Player (despite of what you might see in the Flash Player settings manager's "enable hardware acceleration" checkbox). Firefox does no such thing and this results (at least in my case) in slow/lagging video playback.
Safari does not have any issues either, I guess it does the same automatic hardware acceleration disabling (in the background) as well. My problem was with Firefox that I've the "Better Privacy" extension too which kept removing my hardware acceleration settings.
The solution is to click the "Prevent automatic LSO deletion" in Better Privacy preferences for the
item in the LSO origins list. Now I'm a happy Firefox user again. It's always the small things ...