Default locations for Firefox are the plugins directory under /usr/lib/firefox.<latestinstallversion> OR the plugins Directory under $HOME/.mozilla (the difference is that (of course) the $HOME location is only for you the other one will work for you root your spouse, whoever) It will possibly be easiest to make a soft link to your libflashplayer.so via: (as root) ln -s /usr/lib/firefox.1.0.0.10/plugins/libflashplayer.so\ /usr/lib/mozilla/plugins/libflashplayer.so that should be all one line (without the '\')
If the plugin is already in /usr/lib/mozilla/plugins you'd probably want the link going the other way: ln -s /usr/lib/mozilla/plugins/libflashplayer.so /usr/lib/firefox.1.0.0.10/plugins/libflashplayer.so
