If you leave Firefox open when Windows shuts down, Firefox may automatically re-open with your previously opened tabs the next time you start Windows. Here's how to disable the feature.

Prevent Firefox from starting automatically

1. Type about:config in the address bar and press EnterReturn.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. Search for toolkit.winRegisterApplicationRestart.
3. Click the Toggle button next to it, to set its value to false.

There's no need to restart Firefox; the changes take effect immediately.