There is a default plug-in in mozilla call 'nullplugin' that is run when no appropriate plug-in is found. Try looking for it!
~> locate libnullplugin.so
On my machine the answer is:
and then I think that what the flash player would want is '/usr/local/mozilla'
Sorry, I must have read your reply somewhat sloppy.
If you don't have 'locate' you could use 'find' as fyoder suggests.