I have Thunderbird on Ubuntu 12.04 and every time I open it, it loads twice. Two windows are opened.
To be clear, if I have Thunderbird closed (not minimized) and click on Mail, in the tray bar, it opens 2 main windows.
I have had installed several add-ons, including FireTray and "Minimize to Tray Revived", but now I have removed all of them. Actually I can disable all the add-ons and it still happening.