I’m using now Firefox 56, and 57 developer edition, but when I create a 2d context, drawWindow() function is still not available. The Bugzilla post mentioned by Boris above says something about permissions (“access to canvas drawWindow() with permission…”), but the permission for drawWindow() cannot be found anywhere.
I’m trying to create this context in an extension (tried both background.js and in a script injected into a page), the only result I get is “drawWindow is undefined”. Looking under a debugger at proto:CanvasRenderingContext2DPrototype of a created context object, the only functions starting with ‘d’ are drawFocusIfNeeded() and drawImage()
UPDATE some minutes later:
It works now, all I needed to do was to add “<all_urls>” permission to the manifest.json of my extension. Thanks!