In the future please post which distro you're using so that any community member can give you more accurate solutions directed specifically for your distro.
Figure out where your libflashplayer.so
is located (run commands as root from within the Terminal).
find / | grep libflashplayer.so
Figure out where your firefox is installed.
find / | grep firefox/
I'm going to assume you found libflashplayer.so in /opt/ and you know where your Firefox 3.6.x is installed (at /usr/lib/firefox). Link the flash plugin into Firefox.
ln -s /opt/libflashplayer.so /usr/lib/firefox/plugins/
If you can't find libflashplayer.so then I have provided a download link to it above. Download it, extract it, and move it to your Firefox directory.
tar -xf install_flash_player_10_linux.tar.gz
mv ./libflashplayer.so /usr/lib/firefox/plugins/
If you do not wish to install flash into the system or you do not have admin access then you may install flash into your Firefox user profile which is located in ~/.mozilla-firefox or something like that where ~ is your users home directory located within /home/*. There should be a plugins directory in there.