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.

BIG PROBLEM:
1) The volume, rate, and pitch sliders do not work at all!!
2) If you want to read in another language, say in Spanish, there is no option to do that. Apparently, it allows you to read aloud only in English.

Please look for Android device.

The pause button does not work. Bug reported.

Postscript – if it helps, I find pause not working with at least two other extensions:

https://addons.mozilla.org/addon/native-text-to-speech-tts/reviews/1494393/

https://addons.mozilla.org/addon/text-to-speech-freeapps/reviews/1494400/

Works with default speech voice. Randomly changes from female voice to ugly computer voice. No play speed controls.

I will give 5 stars if you let the user modify the PITCH or SPEED.

他の音声読み上げアドオンよりも使いやすい。ただ、スピード調節機能がつけば言うことないのだが。その点が残念（0.5倍～2倍速ぐらいは欲しい）

The extension is OK but it isn't what I was looking for. I needed an extra option to modify the speed.

Dieses Add-on ist interessant und bei meiner Arbeit hilfreich, leider wird aber beim markieren des Textes die Kopierfunktion deaktiviert die ich parallel dazu benötige.