Rated 3 out of 5 stars
1. "The button in main tollbar has to be added by customizing the toolbar"
2. "The button in message header pane has to be added by customizing this toolbar."
#1 here above can be added, #2 can not, no View HTML button found in the compose window. Is this a bug? Win 8.1, Thunderbird 45.0
Follow-up: I stand corrected. I mixed up message window with compose window. Sorry, I am very new to Thunderbird (2 weeks). The addon works, but this was either way not what I hoped the addon would do.
There has never been a button in compose window. This addon and its buttons are thought for reading messages - not composing.