The MSCONFIG file shows programs (and other services) which are scheduled to load at system startup. But that doesn't tell you which programs and services are currently running.
If you launched the Firefox browser, and then closed it, it's possible that the application file may still be open (running), even though the browser window is no longer visible. (Like other large programs, Firefox doesn't always shut down properly.) Fortunately, it's easy to check on that, and to terminate the application if necessary, by using the Windows Task Manager.
In Windows, right-click on the taskbar (at bottom of screen), and select "Start Task Manager". Then click on the "Processes" tab, and scroll down to see if "firefox.exe" is listed. If so, highlight that entry, and click on "End Process". That will terminate all Firefox processes, so other activities (such as Adobe Flash installation) should be able to proceed.
See also this Firefox help file:
http://support.mozilla.com/en-US/kb/Firefox%20is%20already%20running%20but%20is%20not%20responding
Hope that's helpful.
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