Canvas fingerprinting is a byproduct of the HTML5 Canvas technology that all browser inventors attached to their browsers.
As is the problem with many technologies, it can be used for great or evil, and in the case of canvas fingerprinting it is to follow users.
While there is no primary option available to switch off the Canvas functionality in Firefox, add-ons are possible that block sites from using Canvas. These add-ons, CanvasBlocker and Canvas Defender approach to mind, display information or block requests completely depending on how they are configured.
Mozilla plans to combine a permissions prompt natively in the Firefox web browser. The new focus is part of efforts to introduce Tor protection features or hardening into Firefox.
Firefox displays a confirmation prompt when you visit a site that uses HTML5 canvas image data. This is the case for GitHub for instance, and also on eBay and several other sites.
The message that Firefox displays are subject to change:
Will you allow the site to use your HTML5 canvas drawing data? This may be used to uniquely recognize your computer.
You may allow access or block it, and also use the “always retain my decision” checkbox to make the rule on future visits as well.
The feature is life in Firefox Nightly already currently. Please note that it appears unfinished in the browser version. While you do get a quick to allow or deny site access to HTML5 canvas, options to manage support appears to be missing right now.