Clicking the "X" close button or selecting "close window" from the system tray icon causes the window to close, but when I try to start a new session I get a window saying "Firefox is already running but not responding." I have to use the Task Manager to "end process" the program before I can start a new session. I reported this last week as an intermittent problem, but it has become constant, as in every time. This happened occasionally (once a month or so) in prior releases, but since the 5.0 release it has quickly gotten significantly worse, until now the only way I can close the program successfully is through the task manager.