The latest tarball download of the flashplayer plugin at

http://labs.adobe.com/downloads/flashplayer9.html

has a shell script installer in the extracted directory named
"flashplayer-installer". It is marked as executable so you can run it from
a terminal as follows:

cd ~/download/install_flash_player_9_linux
./flashplayer-installer

Running the installer as above will install it in your home directory which
is OK. To install as root for multi-user use:

sudo ./flashplayer-installer

For firefox, all you need is the flashplayer plugin: "libflashplayer.so".
So you can just manually copy it to "/usr/lib/firefox/plugins".

sudo cp libflashplayer.so /usr/lib/firefox/plugins