Recently, I opened my Firefox and I got the error below
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.When I restart the system this message reappears when I open Firefox Every time

I have kill the existing Firefox process and restart the system but still getting the same error each time. I then decided to delete my Firefox profile to create new one as i suspected that my current profile was corrupted and causing the problem, not sure how though. If you have the same issue you can follow the procedure below
1. Close the Firefox and make sure that it is not running in the background.
2. Open the command prompt and enter any of the following, depending on the Operating System bit
“C:\Program Files\Mozilla Firefox\firefox.exe” -P
“C:\Program Files (x86)\Mozilla Firefox\firefox.exe” -P


3. Click on “Create Profile” to create new profile if you don’t want to delete the existing profile, Click on Rename Profile” if you want to rename the profile and “Delete Profile” to remove a profile from the list of available profiles. You can also delete the associated profile folder and its contents if you select the “Delete Files” option but it is recommended that you choose the “Don’t Delete Files” option unless you are sure that the profile folder contains no valuable data.
4. Click OK to exit the Properties window.
5. Open Firefox, it should open now with the new profile asking if you want to import the setting from IE if you have IE on your system.