For details on what it does and how to disable it see http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins

Thanks, but I read that webpage before asking the question. It contains far too much information for me. As a side-note, I have noticed the flash plugin crashing fairly often in recent months, even with this plugin-container supposedly working to prevent such things. It seems to be quite cost ineffective - - takes a lot of RAM, and returns little benefit.

The plugin-container prevents Firefox from cashing in case a plugin crashes.
It doesn't prevent a plugin from crashing.
So disabling the plugin-container process doesn't sound like a good idea if the Flash plugin crashes often.

Make sure that you allow the plugin-container in the firewall and other security software if needed.