I have installed BT3 to my harddrive not running live I'm sure FireFox has FlashPlayer 9 installed by default? if you want to update to the latest version..
Adobe's website - get (dot) adobe (dot) com/flashplayer
select version to download
.tar.gz for Linux
I agree / save to disk.
Change directory to where you have downloaded the tar.gz file to;
tar xzf install_flash_player_10_linux.tar.gz && cd install_flash_player_10_linux && flashplayer-installer
Press Enter to continue.
Enter Mozilla install path (this is the default FireFox path on BackTrack 3);
Hope this helps...