OS - Windows Vista Home Premium SP2 old Firefox version - 3.1X After upgrading to v.3.6.x, firefox showed up. then upgrade to 4.x. Firefox ran but no window shows up. Try many ways but firefox windows still did not show up. The process is there. 1. Safe mode 2. Profile Manager 3. uninstall/reninstall v. 3.6.17 (Now no windows shown) 4. uninstall/reinstall v.4 This issue happened to me before. That is why I have not upgrade for a long long time. I fixed the problem by reinstalling the old version(I have to find the old version now). Before the upgrade, I manually disabled all add-on and theme, except the default. Please help! Thanks in advance!