The CanvasBlocker extension which has been around for longer than Firefox started implementing anti-fingerprinting measures does exactly this: spoof canvas read

The CanvasBlocker extension which has been around for longer than Firefox started implementing anti-fingerprinting measures does exactly this: spoof canvas readout values. Since blocking Javascript is no longer an option for most people, blocking fingerprintable APIs simply breaks many sites. What we need is frictionless ways to spoof values for these APIs unless the user trusts the site in which case real values can be sent (example the site needs to work and fake values will not be useful to the user).