Don't close Firefox. Simply choose 'end process' for plugin-container.exe. Don't worry about the warning regarding the possible dire consequences of closing processes; it won't do any harm. (You'll probably see a message telling you that Adobe Flash Player has crashed. Simply click the 'page reload' button to restore everything to normal).
Plugin-container.exe is actually there to prevent Firefox fromcrashing. It separates the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins from the main browser, so that if one of the plugin applications crashes, Firefox doesn't. As you've found, it can occasionally cause problems of its own (by hogging the CPU) but it's not a major cause for concern. (Hopefully the problem will be fixed for good when a new edition of Firefox is released).
Chris