I confess: Once in a while I browse #amazon and I (still) do have an account there (even though I criticise the monopoly they build and the way they collect data about their customers a lot). That is the reason why I try not to keep their data in my browser - at the moment #firefox.
At some point there started to appear a pop-up of Firefox very brief when approving the username for login. It disappears after a second or less before the password field is displayed to me.
Here is a capture
As I read the screenshot the use of canvas is allowed for some reason.
I didn't find the reason and am wondering whether it is some trick amazon is using to make @firefox allow the access to canvas or if there is some flaw in firefox.
All in all it looks like I'm allowing canvas access (probably for fingerprinting?) contrary to my wish to deny it in favor of my #privacy.