Canvas works even if privacy.resistFingerprinting.autoDeclineNoUserInputCanvasPrompts is enabled on TBB 8.0, is that normal ?
You can test with this. https://smashcat.org/av/canvas_test/
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If you configure "privacy.resistFingerprinting.autoDeclineNoUserInputCanvasPrompts" to "false" you improve your privacy disabling the automatically inputs don´t requested by user of canvas and do not disable it itself, that is, canvas keeps rolling with reduced chances of fingerprint analysis. The cost of disable is the drop of canvas performance, you can see this in the same test you posted:
With the auto-input disabled the web page always asks if you want to activate it: