There’s a wad of tutorials out there on how to to install this plugin, but I’ve found they either don’t work, or over complicate things.
If you haven’t already found it, download the Flash Player 10 64-bit plugin for linux. Then,
sudo cp libflashplayer.so /usr/lib/firefox-3.0.10/plugins
You might be running a different version of Firefox by the time you read this tutorial, so bust a “locate firefox | grep plugins” to find the appropriate folder.