Why are there so many Mozilla plugins directories?

[Liam Proven]

I seem to find, on 9.04 or 9.10, that there are separate directories for:

/usr/lib/firefox/plugins
/usr/lib/firefox-[version]/plugins
/usr/lib/mozilla/plugins
/usr/lib/xulrunner/plugins
/usr/lib/xulrunner-addons/plugins

Some of these are links to other ones, some are empty.

Most contain symlinks to various things in /etc/alternatives which
then link to actual plugins for media players, java, etc.

Why so many? Wouldn't a single linked or union directory be easier, or
am I missing something important?

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