http://www.cnblogs.com/baizx/archive/2010/08/04/1791759.html#
* /usr/lib/browser/plugins
* /usr/local/lib/mozilla/plugins
* /usr/lib/firefox/plugins
* /usr/lib64/browser-plugins
* /usr/lib/browser-plugins
* /usr/lib/mozilla/plugins
* /usr/local/netscape/plugins
* /opt/mozilla/plugins
* /opt/mozilla/lib/plugins
* /opt/netscape/plugins
* /opt/netscape/communicator/plugins
* /usr/lib/netscape/plugins
* /usr/lib/netscape/plugins-libc5
* /usr/lib/netscape/plugins-libc6
* /usr/lib64/netscape/plugins
* /usr/lib64/mozilla/plugins
# Locations specified by environment variables:
* $MOZILLA_HOME/plugins
* $MOZ_PLUGIN_PATH
* $QTWEBKIT_PLUGIN_PATH