Stop ubuntu updating thunderbird
If you don't uncheck that starting Quick Books will start Thunderbird.
 If none of this helps you need to figure out what application started it.
The Ubuntuzilla project's repository contains repacks of the unmodified official Mozilla builds of Firefox, Thunderbird, and Sea Monkey. But the process to uninstall Thunderbird varies depending on the operating system you are using.This chapter looks at the steps for uninstalling Thunderbird on Windows, Mac OS X, and Linux.When you logout/shutdown next time, uncheck the option that says "Save Session" in the pop-up that appears.This will ensure that the session is not saved and none of the programs will auto-restart.