Can I open a tutorial lesson in a new browser window or tab instead of the pop-up window?

You can, but then you will not be able to mark the lesson complete. There is a "mark complete" button at the end of each tutorial lesson. That button will only work if the lesson is opened in the pop-up window that appears when you click the "open lesson" button. If you open the lesson in a new browser window or tab, the "mark complete" button will still appear, but it won't do anything when you click on it.