Flash plugin - where?

[Timothy Murphy]

When I install Flash plugin (or other plugins)
by going to <https://addons.mozilla.org/>
and following the instructions,
the plugin is installed in /usr/lib/mozilla/plugins/ .

These plugins are not listed when I enter "about:plugins" in Firefox,
and they do not appear to be active.

However, if I link these plugin files to .mozilla/plugins/
then they are listed and are active.

Is this normally required?
Or have I omitted some step in installing plugins?