TL:DR - I personally find MS Zira voice with 75% speed, 100% pitch, to be good enough for most scenarios. Default settings sound horrible, but can be tweaked, that is what I mean.

Okay overall with all things considered, this is currently among the most easy to use TTS (Text to Speech) addons

Pros:
- Easy to use, works fine with a google translated page, easily identifies what you want to read

- You can use Google Wavenet, Amazon Polly & IBM Watson voices with it, if you have API access (all 3 of them have a free tier)

- Has a dark background pop-up that shows the text in a simple readable format alongside speech

Cons:
- Biggest con is that it doesn't use the new Microsoft voices that come with Windows Narrator. It only uses the old Zira, Adam, voices which are of lower quality.

- No way to add a custom Offline voice model. The option of 'Custom Voice is for the online APIs'

- I heard somewhere, that even in offline mode, it does uses some google apis, but haven't checked myself. Given it is recommended by Mozilla officially, I guess its alright for non-security scenarios.