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firefox closing problems
After upgrading to FF4 I've found that while clicking on the button to close the program makes the window disappear FF itself continues to run for anything up to a minute and a half and multiple entries (6 or 7 is not uncommon) of plugincontainer.exe run hogging all the memory. I have to close the program via the processes tab in the task manager
Alle svar (1)
Your system specs point to several possible causes. To start with, your graphics driver dates from 2009. The latest WHQL version is v266.58 released in January 2011, download from http://www.nvidia.com/
Uninstall the existing drivers via Device Manager rather than Add/Remove Programs. To get to Device Manager, go to Start | Run, type (or copy/paste from here) devmgmt.msc and click OK.
Open the tree to Display Adapters and then double click your 9400 GT to get the dialog box on the right of picture. Then click the Uninstall button in the Driver tab.
When the system reboots, run the driver installation file you downbloaded earlier.
Plugin_container.exe is a kind of sandbox which runs within the browser and isolates certain plugins like Adobe Flash if they crash. Terminating the process will cause the whole browser to crash or hang. Try using Adobe's Global Security Panel to adjust settings @ Adobe Global Settings Manager
See also: Firefox hangs or is not responding - How to fix
You could also try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.