Very useful Addon, I use it frequently to save articles I like, but have an ugly layout.

One feature I would really like to have is the option to format everything to justified text.

There are some features in design mode that don't seem to work - an explaining tooltip would be helpful (or did I miss the description somewhere?):
The crossed-out T seems to toggle Bold/Italic mode, but I can do that with the B and I buttons as well.
The picture symbol does not seem to do anything at all?
The quote-symbol seems to do the same as the text-indentation, except you cannot undo it.

And finally, I'm missing the 'options'-page, as shown in the video, is this feature not available any more?

I never use the text-to-speech function so I cannot say anything about it.