Plugin-container is a Firefox process designed to ‘out-source’ the plugins used by the browser. Firefox’ model is based on the use of plugins to add functionality, however this goes along with a chance of instability due to a malfunctioning plugin – this would cause the whole browser to collapse since the plugins were all included in one central Firefox process. With the implementation of the plugin-container, the plugins are seperated from the browser process, making it more stable. Now if a plugin crashes, Firefox remains unharmed.
It is known however that this feature causes serious performance issues with many users, that’s why it might be a good idea to test what happens if you deactivate plugin-container (the plugins are not disabled that way, they are just stitched to the main process again).
To do so, open Firefox and direct the URL line to about:config. This is the internal configuration of Firefox, be sure to take the warning that will be shown seriously.
In the search bar on top, enter dom.ipc. A few boolean entries will be shown whose name begins withdom.ipc.plugins (on Firefox 9 there are two, while there are five on some older versions).
Set all of them to false to disable plugin-container: