Firefox Portable Edition Already Running Error

I know someone out there might have same error experience in his/her past and I would like that person to share the experience with me so that I can get rid of this error.
Another instance of Firefox is already running. Please close other instances of Firefox before launching Mozilla Firefox, Portable Edition (Test).
At the startup screen I run Mozilla Firefox explorer but it didn’t got started and didn’t even show up so I installed the Portable edition of Firefox and then open its .exe file and by that time it gave me this information message that Firefox is already running.
I don’t see any Firefox running then why do I receive the same message again and again please solve this issue for me.
