This add-on allows users to prevent websites from using the Javascript <canvas> API to fingerprint them. Users can choose to block the <canvas> API entirely on some or all websites (which may break some websites) or just block or fake its fingerprinting-friendly readout API. More information on <canvas> fingerprinting can be found at http://www.browserleaks.com/canvas.

The different block modes are:

• block readout API: All websites not on the white list or black list can use the <canvas> API to display something on the page, but the readout API is not allowed to return values to the website.
• fake readout API: Canvas Blocker's default setting, and my favorite! All websites not on the white list or black list can use the <canvas> API to display something on the page, but the readout API is forced to return a new random value each time it is called.
• ask for readout API permission: All websites not on the white list or black list can use the <canvas> API to display something on the page, but the user will be asked if the website should be allowed to use the readout API each time it is called.
• block everything: Ignore all lists and block the <canvas> API on all websites.
• allow only white list: Only websites in the white list are allowed to use the <canvas> API.
• ask for permission: If a website is not listed on the white list or black list, the user will be asked if the website should be allowed to use the <canvas> API each time it is called.
• block only black list: Block the <canvas> API only for websites on the black list.
• allow everything: Ignore all lists and allow the <canvas> API on all websites.

At present, only my domain (kkapsner.de) is whitelisted by default.