This Addon does not have an icon that is always visible. Only a fingerprint icon is displayed in the adress bar when some canvas data was faked for the web page. You can test at http://kkapsner.github.io/CanvasBlocker/test/test.html
If you still think that it's not working properly try version 0.4.4a and open a new issue at https://github.com/kkapsner/CanvasBlocker/issues
