First you need a 32bit firefox.manual installation method
using the official flash player
Open a terminal ( Applications Accessories Terminal )
tar xzvf install_flash_player_10_linux.tar.gz
###### this command will run as root, any mistake at this point could break your install
sudo cp libflashplayer.so /usr/lib/firefox-3.0.10/plugins
Close any firefox windows you might have had open and open a new firefox.
Go to youtube and see if it works.
The Linux version of crash
player, err, flash player can cause firefox to slow way down and can cause it to crash.