I love this so far, Canvas fingerprinting definitely needs to be addressed. The one thing I would change is having a gui allowing us to whitelist or blacklist domains as we encounter them. Something similar to Noscript maybe. At least having an icon we can place would be great. (unless there's one I can't see, I'm on palemoon. If that is the case, disregard this comment)
Also, it would be nice if instead of a popup in the middle of the screen, the warning would be a more subtle bubble in the corner, similarly to self-destructing cookie's messages. Again, having options to whitelist or blacklist for the future on this message would be great.
As far as the canvas elements themselves, more information would be great. Things such as the source of the script, maybe with a "details" button with more technical info.
Overall though, great addon. Good luck in it's development and thanks for addressing a privacy issue. I hope my suggestions are helpful. I do understand that this is an early version and some of those Gui elements might be planned already for the future, so no worries there. :)
Browser: Palemoon 24.7.1
OS: Linux Mint 17, Xfce