http://www.ghacks.net/2010/06/25/what-is-the-process-plugin-container-exe/
http://www.ghacks.net/2011/10/27/why-are-2-plugin-container-exe-processes-running/
Plugin-container.exe basically is Mozilla’s attempt of separating core plugins from being executed in the same process as the Firefox web browser. This is done foremost to improve stability. A plugin crash previously would have often causes the whole browser to crash as well. With plugins moved to plugin-container.exe the browser remains unharmed if a crash happens.
You can test it on your system easily. Make sure you have the Adobe Flash and Microsoft Silverlight plugin enabled. Visit YouTube and start watching a video to trigger the Flash plugin-container.exe. Now open a new blank tab and visit a site that uses Microsoft’s Silverlight technology to play videos. You will notice that plugin-container is launched twice and that the processes stay open when the video stops playing and even if the page is closed.
Don't do this, Firefox may crash if plugin crash & take down Firefox,
Firefox users can disable plugin-container.exe completely in the advanced configuration of the browser. Enter about:config in the address bar and hit enter. Now filter for the following term and set their values to false to disable the feature. [Don't]
dom.ipc.plugins.enabled
You can also disable separate processes for a specific supported plugin instead.
dom.ipc.plugins.enabled.npctrl.dll (Microsoft Silverlight)
dom.ipc.plugins.enabled.npqtplugin.dll (Apple QuickTime)
dom.ipc.plugins.enabled.npswf32.dll (Adobe Flash)
dom.ipc.plugins.enabled.nptest.dll (NPAPI test plugin)
http://www.ghacks.net/2010/05/12/mozilla-plugin-check-now-checks-plugins-in-all-web-browsers/
Mozilla Plugin Check Now Checks Plugins In All Web Browsers
The Mozilla developers introduced the Mozilla Plugin Check back in October 2009 as a way to inform users of the Firefox web browser about outdated plugins. Plugins that are not up to date can pose both a security and stability risk.
Three different statuses are available: Up To Date confirming that the latest version of the plugin is installed, Update suggesting that the plugin is outdated and Research if a plugin cannot be identified yet by the service.