After upgrading to 4.3 (or 5 later) firefox does not start for one user of my pc, while it works for the others. I start it, I see a running process with 50% CPU but the browser window never appears. There is no error in the event viewer. I tried with a different user of my pc and it worked fine. I logged again with my main user and disabled all startup items from msconfig but still firefox window does not start and a firefox process remains running.