Open the page you’d like to test in Safari, rather than Firefox or Chrome. The combination of VoiceOver and Firefox, in particular, can be buggy.
Turn on Voiceover. There are several ways to do this:
Press Command + F5, or
System Settings > Accessibility > VoiceOver, or
Ask Siri to do it :-)
Press Control + Option + U to pull up the rotor. Use the arrow keys to confirm that all the headings, links, forms, tables, and landmarks are as you expect.
Use the rotor to navigate to the specific area that you’d like to test.
Use these keyboard shortcuts to navigate and interact with the area you are testing: