When you close Firefox and want to reopen it, the following message appears :
"Firefox is already running but is not responding. To open a new window, you must first stop the existing Firefox process, or restart your system."
Cette erreur se produit lorsque Mozilla Firefox devient instable et qu'il prend beaucoup de mémoire vive (RAM).
To restart Mozilla Firefox, open the Task Manager by pressing "CTRL + ALT + DEL".
If you're running Windows Vista, 7 or 8, you must then click "Open Task Manager".
Then, go to the "Processes" tab, select the process "firefox.exe" and click the "End Process" button.
Then, click "End Process".
Wait a few seconds and the process "firefox.exe" should disappear from the list.
If this is the case, you can run "Mozilla Firefox". Otherwise, restart your computer and restart Firefox.