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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.
And when you do this, you are now more fingerprintable than you were with resistFingerprinting off, as the specific combination of anti-fingerprinting measures and canvas-enablement makes you more unique. Which is why it's hidden in about:config.