Often when I try to open Firefox again after closing it, a message pops up saying Firefox is already running (it is not!), so I have to restart my computer.

Many times when I try to open my Firefox (3.6.11) after closing it, a message pops up saying "Firefox is already running. You must close the porcess or restart your computer before you can open it." Because it is already closed and I have nothing else to close, I close the message and try again, and the same message pops up again... So I have no choice but to restart my computer. (charlie181818@*******.***)