Firefox wouldnt work?!

I get this message everytime I try to access it:
Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

I clicked it multiple times and the message still pops up. I tried closing ALL windows and programs, I tried restarting, but nothing worked. What is happening?

P.S. I also scanned my computer and there were no viruses.

I googled the error message and found this.
Helpful?

Incorrect.

This error message pops up, because the Firefox process is running in Task Manager.

Ending the process in Task Manager will allow you to open a new window.

Instead of trying to open a new window via the Firefox shortcut (if you already have Firefox open), use File > New Window.

That should help repair that issue from happening again.

It is a Firefox things.