Firefox sometimes after quitting will remain in memory, however seem to be closed. See: [[Firefox hangs]]

Create a new profile as a test to check if your current profile is causing the problems. See "Basic Troubleshooting: Make a new profile": *https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files. See: *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox See also: * https://support.mozilla.com/kb/Firefox+is+already+running+but+is+not+responding