Thunderbird won't close properly and I can't restart the program. Any new ideas?

I keep getting the error, "Thunderbird is already running, but is not responding. To open a new window, you must first close the existing Thunderbird process, or restart your system", when I close and then later try to restart Thunderbird. The only fix is to either open Task Manager and kill Thunderbird that way, or to restart the computer. I have searched the knowledge base on Mozilla online and have already deleted the Lock files from the Profile directory but this did not fix the problem. I also already went into Safe Mode and tried closing and restarting the program there. The problem does not occur in Safe Mode. Only in regular operating mode. I am running Windows 7 64 bit. Any help would be greatly appreciated.