Plugin-container.exe is a process which is designed to prevent Firefox crashing when an individual component within it (such as Adobe Flash) malfunctions. I disabled it with Firefox 4 but (despite occasional problems) I've not found it necessary to do so in Firefox 5.
If you want to disable it, type 'about:config' in your address bar (and click 'Go'). Read the warning about dragons and, if you're happy to proceed, click the button to enter Firefox's hidden settings.
Scroll down to the line which reads 'dom.ipc.plugins.enabled'. Double-click on that line, to change 'value' from 'true' to 'false'. Then go to the next three lines in the list and double click to change 'false' to 'true' in each case.
Chris