I saw, in some old threads on this forum, that a few people were having a problem in which they received the message:
"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 was having this problem too. At first, I couldn't figure it out, but then I did figure it out later. So I'd like to share what I found.
First, I tried the suggested solutions from this forum. One person said that this problem can arise if you rename FirefoxPortable.exe to firefox.exe. But I had never renamed anything. It was also mentioned that the user should check the Task Manager. However, I checked the Task Manager and didn't see anything like "firefox.exe" or "FirefoxPortable.exe" listed in the Task Manager.
Then, I figured it out. I was at my workplace when this happened. At my workplace, all the computers in the building are set up so that any user can login to any computer in the building with their username and password. After logging in, the user has access to their own personal files, from any computer in the building. Without realizing it, I was logged in to two computers at the same time. I had Firefox Portable running on one computer. Then, while that was running, I was logged in to the second computer, and I was trying to start Firefox Portable there. That's when I got the error message.
So, apparently, Firefox Portable detected the instance from my first login, but the Windows Task Manager was unable to detect that process.