Hardware acceleration for flash is not enabled by default. You can enable it as follows
the last command creates a new document mms.cfg, enter in it
sudo mkdir adobe
gksudo gedit mms.cfg
save and restart Firefox.
It may work for you, but in my experience flash crashes almost every time with hardware acceleration enabled and it doesn't work for most flash sites (it seems to work only on Youtube if it doesn't crash). If that happens just remove the folder /etc/adobe. I have made a script to switch between hardware acceleration enabled and disabled but pretty much leave it disabled.
EDITED: There is another type of hardware acceleration for playing videos called vdpau for Nvidia. To enable it you need to install extra packages: libvdpau1 and vdpau-va-driver.
mplayer and XBMC support hardware decoding with vdpau. For mplayer use any of the guis (gnome-mplayer or Smplayer) and set output to vdpau. Vlc also supports it since 2.1.1 but not sure if it is enabled in any of the Ubuntu builds including ppas (ffmpeg in ubuntu is too old). vpdau has nothing to do with your original question but just mention it since you have a Nvidia card.