When I push Alt and F4, it often occurs that the window closes but the process does not stop. After I closed Firefox and wanted to start it again, it opens only every second or third time without error messages. When it does not open properly, it shows following error message, "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system." When I have a look at the task manager, it shows that the process firefox.exe works, but doesn't show any activity (FYI, the memory usage is normal). The process is shown there as a background process (FYI, a Win 8 feature, and background processes are only visible in the detail view of task manager). When I stop the task, Firefox opens regularly. So, the problem is that when Firefox should be closed as the windows get closed, the process doesn't stop. That problem happened quite rarely with the previous version of Firefox, with Firefox 29, it transpires as often as described above.