(In reply to sanketh from comment #8)
Even if the prompt is shown and the user allows access, there is a UX hurdle where the user may need to reload the page for the setting to take effect. See Bug 1633813 I wonder if this is part of the problem, where the users toggle the prompt but don't reload but after they reset the config, they reload the page and it is the reload the solves the problem?
(Huh. I replied with the below message via email three days ago, but I guess it was never added)
This sounds like a reasonable explanation.
I wonder if we can provide a better UX by being less precise. Instead of
prompting at the time of extraction (ex. calling getImageData), we
prompt when the first canvas element is initialized on the page. The
prompt says something like "Will you allow <website> to access
information about graphics in the future that could uniquely identify
your computer?". If the user allows it, then extraction is permitted,
otherwise the site gets randomness. When permission is denied, Firefox
could even keep a counter of the number of times the site received
randomness and show it in the ETP display (but that is more in Johann's
Really, we need a standardized async API for this.