Could you post the specific error message you got? Sometimes when you close Firefox it does not unload completely from memory; then you get the message that Firefox is still running. This article has some potential solutions for this problem: https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding#w_firefox-hangs-when-you-quit-it

Which security software (firewall, anti-virus) do you have? Did you check your security software (firewall)? ---- Do a clean (re)install and delete the Firefox program folder (C:\Program Files\Mozilla Firefox\). Download a fresh Firefox copy and save the file to the desktop. *Firefox 13.0.x: http://www.mozilla.org/en-US/firefox/all.html Uninstall your current Firefox version, if possible. *Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you will also lose your personal data like bookmarks and passwords from profiles of other Firefox versions. Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer. *It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling. *http://kb.mozillazine.org/Uninstalling_Firefox Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall