In about:config you can only choose 'true' or 'false'.
There is no 'toggle'.
The highlighting works, but there is no sound/voice.

"Note 2: HTML5 TTS API is still a work in progress and may encounter crashes or errors. If the API is not active in your browser, please go to "about:config" page and search for "webspeech" keyword. Once you find "media.webspeech.synth.enables" item, right-click on it and click - Toggle - to enable it."

VERY NICE !! ONLY I WISH IT HAD AN SPEED CONTROL & GIRL VOICE :D

Terrible UX - opening a separate window for a seemingly simple function defeats the whole purpose of an addon. Waste of 5 mins of my life (including 2 mins reviewing this).

Russian language - not working at all

Het gebruikt de TTS van windows. Dat werkt, maar die automatische taal herkenning???

There is no speed or pitch options, so it's extremely slow. It is opens up into a big pop up that when you click off of it goes away which is also very annoying.

Does not work. Cannot get tts to start.

Hard to listen to, unnatural sounding, horrible quality. There it is a reason it is (currently) and 2 star program

Horrível! A voz é masculina e robótica! Não possui controle de velocidade da voz.

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.