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Since I’m the idiot learning this by filing bug reports, I thought I’d pass it on to all other #FireFox users on here. If you enable resistFingerprinting in about:config, the web site will start showing you weird stripy graphics anywhere an HTML5 canvas graphic would be generated...
My apologies to the Bugzilla team for wasting their time holding my hand on this one. Would have honestly never noticed the little "HTML5" info icon to the left of the URL bar though without their help.
The resist fingerprinting mode scrambles canvas read out to prevent sites from using it to create a fingerprint. Because of that, any site that needs to read the canvas back for legitimate purposes will also receive scrambled data. You get more privacy for the minor inconvenience of having to manually allow canvas usage for the sites that actually need it.
Yes I’m aware, but the user interface doesn’t make it easy to understand A) why the canvas looks scrambled, and B) how to permit it on a per-site basis.
The technical implementation is fine, it’s just the user workflow that needs fixed.
Not really, because doing this will make you more fingerprintable (see my other comment). That's why the default settings are striking a balance between making all users look similar and not breaking too many things (that would cause users to use user-specific overrides that make them more unique), and why resistFingerprinting is in about:config rather than a user-facing setting or enabled by default.