It works, that's nice. But it appeared to be too cumbersome to use.
Clicking the plugin icon in the toolbar did nothing here (FF58). (Why is it there then?)
The only way I could quickly find to make it speak:
1. Select text. (Perhaps this is optional.)
2. Right-click (selected) text.
3. Select Open TTS Window from the context menu.
4. Click Play in the popup.
I've been spoiled by "Read Out Loud" in Chrome, so I uninstalled this one, sorry.