According to wikipedia

Canvas fingerprinting is one of a number of browser fingerprinting techniques for tracking online users that allow websites to identify and track visitors using the HTML5 canvas element instead of browser cookies or other similar means. The technique received wide media coverage in 2014 after researchers from Princeton University and KU Leuven University described it in their paper The Web never forgets.

https://en.wikipedia.org/wiki/Canvas_fingerprinting

Also if you just google “librewolf canvas disabled”, their FAQ shows up with the exact reason why https://librewolf.net/docs/faq/#should-i-allow-canvas-access-how-do-i-do-it