Firefox settings. Fingerprint resistance (privacy.resistFingerprinting) will auto-reject the canvas and show a relatively non-intrusive icon in the address bar that lets you re-enable it for that website[0].
Since webGL requires you to use canvas, this should also block that attack, although I'm currently going a step farther and disabling webGL entirely (webgl.disabled), since I've seen a few sites (panopticlick for example) get around the prompt specifically with webGL, and I don't know how they're doing it.
Firefox's fingerprint resistance efforts are showing a lot of promise, although you will have to put up with some quirks (like learning to read UTC time).
I know there's a prompt for canvas, but it doesn't seem to work for webgl. On a fresh install with fingerprint resistance enabled, webgl fingerprinting appears to work fine[1].
what extension does this? there's plenty that disables it outright, but is there one that shows a prompt?