Portage just updated firefox-bin to 21.0, and after that I had no firefox plugins.
Finally I discovered that the new version apparently ignores the traditional symlink /opt/firefox/plugins->/usr/lib64/nsbrowser/plugins I worked around it by creating my own symlink ~/.mozilla/plugins-> and all the plugins are working again. Maybe this will save some time for you. Dunno if this a bug or not.
