When I click on the Firefox icon to get to the internet I get a message saying that Firefox still has a window open and that I need to close it before I can open another window even though there is no window open.

I have not been able to get onto the net through Firefox for several weeks. Each time I try to get on, I get this exact 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." If I restart I get the same message. I really like Firefox so I'd like to solve this situation.