Thanks for any assistance you can offer to resolve. This error message began appearing two days ago and I can't figure out why, or how to resolve. Occurs any time a message header is selected by clicking with mouse. Popup comes up twice each time and closes by selecting "Okay". After second popup and selecting "Okay" the message opens in the lower reading pane. Process begins again when you select a new message--even the last one you opened. Again, thank you for your time! Love Thunderbird otherwise and have had no other prior problems.