NVDA is a popular open source screen reader on Windows that works well with Firefox. It's similar to JAWS, another popular Windows screen reader that works best with IE. But NVDA has no licensing fee plus an open bug tracker, making it great for users as well as developer testing.
In this lesson, you'll learn how to browse a webpage using NVDA's keyboard shortcuts, including headings navigation, skip links, and the Elements List. You'll also learn a bit about Forms Mode and how it impacts screen reader navigation. For more tips, check out the resources below.
If you're on a Windows machine, you can install NVDA directly. If you're on a Mac or Linux, you could install a Windows virtual machine: