The "Minimize to Tray" addon for Thunderbird didnt support a "Minimize at startup" option (minimizing right after execution) so i found this:

Making a Startup Application minimize after opening.

To make an application start up when Windows is launched, you simply drag its shortcut into the StartUp menu. If you want this application to minimize as its opened so that you don't have to minimize it later you can simply tell Windows to start the program minimized.

If you already have the program in your StartUp menu all you have to do is change its Properties. To get to the shortcut:

• Right Click the Start button.
• Click on "Explorer"
  
• Double-click on "Programs" (right side).
• Double-click StartUp, and there are the shortcuts.
• Now, you simply right-click the shortcut and select Properties.
• Click the Shortcut tab.
• In the Run box select Minimized and click Apply.

The program will open up minimized upon starting Windows from now on.

Source: Computertips.com