The original browser plugins (before installing flash9) are in /usr/lib/browser/plugins/

The Mozilla plugin directory is /home/user/.mozilla/plugins/

You don't need to copy the flash player plugin to the mozilla dir, you can just creat a sym link,
ln -s /usr/lib/browser/plugins/libflashplayer.so /home/user/.mozilla/plugins/libflashplayer.so

Backup your original flash plugin before installing the flash 9. That package simply overwrite the old one.

I couldn't get YouTube to work properly in OS2006 with flash9, it's all scramble. It also crashes a lot more.