The Security forum discusses installation problems, including installing plugins.
See also
http://plugindoc.mozdev.org/notes.html#loc-unix which states:
On Unix, Mozilla and Mozilla Firefox search for browser plugins in the following locations:
* The path pointed to by environment variable MOZ_PLUGIN_PATH
* Your Mozilla plugins directory (For example, /opt/mozilla/plugins)
* ~/.mozilla/plugins (Mozilla Firefox looks here too, see Bug 172891).