Thank you for the link, I bookmarked it to read more in detail. You're right, that's a nice overview of the features. I've always felt the need to learn more about accessibility (and regularly apply it to what I build). This looks great for the purpose.
I found in the browser preferences that I'd checked "Prevent accessibility services from accessing your browser". I had to uncheck it to be able to enable the Accessibility Inspector. Will be exploring it more.
It should work though once you click on that button, weird that it doesn't seem to be working for you.
Edit: Just read through more of that link, it's got quite a good breakdown of the features on it.