This addon does what is advertised, and does it well.
However, I had to remove it. I suspect the functionality that discards irrelevant content is to blame:

When using Wikipedia, >any< article on Wikipedia, the Table of Contents would appear only so long as the page is still loading. The moment it's done loading, the Table of Contents disappears, the article is displayed as though it had never been there.

Since Wikipedia is one of my most frequently used bits of internet, this made Pericles worthless for me.

I originally encountered the problem in Waterfox 56.2.5, BUT it replicates perfectly in Firefox 64.0. That is to say, tables of contents on Wikipedia disappear in Firefox when Pericles is installed, too.

Two stars, because it does the text-to-speech well, but no more than two because it does not handle recognizing irrelevant content well.