My firefox didn't respond when I wanted to download a picture from photos.google.com. Then I clicked x to close its window which has multiple tabs open. Then I wanted to start firefox, but it didn't respond. I find there are several related processes in the output of ps aux | grep -i firefox, but kill their pid doesn't work. How can I remove the firefox-related processes, so that i  can restart firefox?The statuses of the firefox-related processes are D, Dl, and S+. What do they mean respectively? My os is Ubuntu 14.04. Thanks.
Why can't I kill the firefox processes?
process