Start Mac in safe mode

Still In Mac safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Does problem go away? Please reply to let us know results.

oddier said

Thunderbird is frozen on my Mac. Can neither "Quit" Thunderbird nor "Shut down" the computer.

Hi Mery. I already found the solution by myself. The only thing I had to do was to "Force Quit" the program. Thanks for your answer. Best wishes, Oddier