The extension is dead. I reported it to Mozilla. The site easypronounce.com has been sold and became fastemoji.com

And when I test the addon, it's not a surprise: Doesn't work, I select a word, click on the addon button, but nothing is shown, and nothing happens when I enter a word in the window.
The https://www.fastemoji.com, it's a search engine for emojis, not a search engine for words pronunciations!

Strangely, the extension is called "How to pronounce words" in the extension manager.