Thunderbird: How to Open a Dialog Window

window.openDialog(
"chrome://editor/content/EdFormProps.xul", "_blank", "chrome,close,titlebar,modal");
window.content.focus();

Found this in line 2511 of ComposerCommands.js. Could be useful when designing some options menu for your particular extension.

If you were designing some kind of options window, you would replace EdFormProps.xul with your own .xul file.