It's great that this was the first really good TTS extension for Firefox, however as others have pointed out it is quite buggy.
The most serious bug is that having it enabled makes most website GUI buttons behave wrong or not at all. I can only use the extension if I enable it. Refresh the page that I want read aloud, and then disable it after. That means that I only use it for long texts where I am going to stop everything and listen to it read.
Regardless, thanks for your efforts in being the first to create a decent TTS app. That is a milestone.