hello, it might be normal that the browser process needs another 2-3 seconds to shut down in the background after all visible windows are closed. it shouldn't take much loger though - you might monitor in the windows task manager (ctrl+shift+esc in the processes tab) how long it takes for firefox.exe to be removed from the list.

to speed things up, you might want to reset firefox to the default settings, since extensions can be a factor in the slow shutdown: Refresh Firefox - reset add-ons and settings

in addition & probably not related to the issue you've described: please update your plugins (you java version is seriously out-dated & has security vulnerabilities that are actively exploited on the web).

See also:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

Use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar.