Excellent add-on, shows all the sites that try to manipulate canvases, and stops them from doing it. You can also set a whitelist, note that some Google pages are whitelisted by default!
One bug: this add-on causes a memory leak in Firefox 32: after browsing for a while you will see in about:memory a lot of ghost windows, which also make the browser laggy as the garbage collector repeatedly tries to clean them up. Temporarily disable this add-on (no restart required) and the GC will shortly get rid of them. This should be fixed though.