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These canvas-context properties are Mozilla-specific, and has obviously never been implemented anywhere else. Currently they are in the process of being removed, see [bug 1782651](https://bugzilla.mozilla.org/show_bug.cgi?id=1782651) and [bug 1294360](https://bugzilla.mozilla.org/show_bug.cgi?id=1294360), which in practice means that in e.g. Firefox Nightly the `addContextCurrentTransform`-function is now being used in the *built-in* PDF Viewer (which was obviously never intended). We should thus be able to replace these Mozilla-specific properties with `CanvasRenderingContext2D.getTransform()`, which is available in all browsers that we currently support: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/getTransform#browser_compatibility