Firefox plugin

Can anybody tell me why there is more than on plugin folder for the firefox?
/usr/lib/firefox/plugin
/usr/lib/firefox-addons/plugin

And there is a symbolic link in /usr/lib/firefox-3.0.6 which points to the /usr/lib/firefox-addons/plugin, which is btw. empty. I needed to add manually a symbolic link from /usr/lib/firefox-addons/plugin to the openoffice plugin, the same one that exists in the /usr/lib/firefox/plugin to get that plugin be known by firefox-3.0.6. The quesion is why to have more than one plugin folder and which one is the right one? If i want to keep two version of the firefox togeather will the /usr/lib/firefox/plugin become inconsistent?