Seamonkey asking for additional plugins

I have installed Seamonkey from a /usr/ports build. I have seen a status bar displaying
Additional plugins are required to display all the media on this page
There is no indication of what plugin is needing, or what is not displaying.
On this forum, it only comes up when I type a response to a topic, so is showing right now as I type this.
There is a hot link to a Preferences page, that has "Activate all plugins by default" checked, and no indication why Seamonkey is complaining.
This is on a default installation without any add-ons installed. And as with Firefox, the installation does not create plugins directories, either globally or locally.
What am I missing.
This forum has a "Flash-based uploader" option which can be disabled in your Preferences settings.