We resolved this issue by opening the task manager and manually ending Mozilla under processes, not applications, since it was not on the applications list, but was on the processes list.
He says this happens to him every time he closes Firefox and tries to reload. (different locations, different networks, different brands of laptop.) I witnessed first-hand that he had the same error message, only his did not go away, nor could we get his browser to open. However, I now just found out that my cousin has been dealing with the same problem, only worse, when he uses his laptop at home. Usually, after receiving the error message I just wait a few seconds and try again and the browser opens just fine. I have been getting this on my laptop for a few weeks now, off and on, when I close Mozilla Firefox for whatever reason and try to reopen it immediately after. To open a new window, you must first close the existing Firefox process, or restart your system."
"Firefox is already running, but is not responding. I was just curious if anybody has been receiving the same error message: