The plugin container is meant to use as much memory and CPU power as the plugins need. In basic terms, any memory and CPU cycles used by the plugin container would have been used by Firefox itself in previous versions. So you have to judge if the amount of RAM and CPU taken by the container is disproportionate to the amount of RAM and CPU take by Firefox in previous versions.
That said, you shouldn't need to have any sort of technical knowledge to judge this, and you shouldn't have to worry about the technical details at all. If the plugin container is consistently using up RAM above 50MB or 100MB, that's most likely a problem. However, it's most likely to be a problem with an add-on (probably one of the plugins in use) rather than Firefox. Run Firefox in safe mode, select Disable all add-ons when you start safe mode and see if the problem disappears. If they do, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and themes and this one for plugins.
If you need support for one of your add-ons, you'll have to contact its author.
If the problem is not disappears when all add-ons are disabled, please tell me, so we can work from there. Please have no fear of following my instructions to the line, as all can be easily undone.
I have consistently had this problem with just about every release of firefox, and can tell you that the plugincontainer.exe file is the culprit, not addons. I have run Fox stripped of addons and had the same exact problem. When I go in to the configuration settings and change the values to false to prevent it loading, and (on mine anyway) set the timeouts to 10 seconds, it cleared the problem up.
Many others have said the same thing.
So my question is, why doesn't firefox just get rid of plugincontainer.exe? DUH!