Not criticising _intended_ functionality.
I haven't used it much - I tend to use Read-Aloud, but it breaks the ability to change font in Gmail: select text, click to choose font, text is unselected, so whatever font is chosen is not applied. Disable this add-on, and I can change font in Gmail.
FINALLY found out why! Bye!

Voz robotizada e fala pausada . Português ruim.

"Access your data for all websites" why need this permission if other text to speech extensions doesn't require it???

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.

Doesn't work.

No funciona simplemente.

because not hebrew.

This is a good addon. I don't know why it is underrated. Maybe it's because sometimes it doesn't speaks whole sentence but word by word. There is also a small bug that if I enabled the addon, I can't not use drag-text function which is original in Firefox. Hope it will improve.

might be one of the worst extensions ever for TTS. it hurts the ears and is hard to understand.