It's chrome-only; having it in 'window' is breaking things. See https://bugzilla.mozilla.org/show_bug.cgi?id=486200#c28 ff.
Created attachment 392185 [details] [diff] [review] fix Easy stuff...
Created attachment 392186 [details] [diff] [review] extra test This test will make sure that no-one repeats my mistake.
Added some documentation to https://developer.mozilla.org/en/Firefox_3.6_for_developers, it isn't enough, but at least it's something :)
At least this part is complete. https://developer.mozilla.org/en/nsIDOMWindowUtils
There are still a confusing bunch of references to window.mozScreenPixelsPerCSSPixel on MDC.
Updated: https://developer.mozilla.org/en/DOM/window https://developer.mozilla.org/en/Firefox_3.6_for_developers Deleted the article on DOM/window.mozScreenPixelsPerCSSPixel.