Normally Firefox unloads completely from memory within about a minute when you Exit it. The message from the installer may indicate that it didn't shut down completely in the allowed time. You might want to shut down and restart Windows and try again without starting Firefox. Or you could try unloading Firefox using the Windows 7 Task Manager (Ctrl+Shift+Esc). First check the Applications tab, and if you find Firefox, select it and try End Task. Next, check the Processes tab for firefox.exe. If you find it, select it and click End Process. Then try the installer again. Success? If you need a new installer at any point: https://www.mozilla.org/firefox/all/ (scroll down to your preferred language)