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Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
This could be an issue with the thunderbird install, or a permissions issue.
Would you like me to connect to your mac from here, look into the issue and repair it personally?
Or would you prefer to have me try and guide you via written support instead?
Thank you for your response,