Remove mozOpaque.

web/viewer.js

@@ -2112,7 +2112,6 @@ var PageView = function pageView(container, pdfPage, id, scale,
 
     var canvas = document.createElement('canvas');
     canvas.id = 'page' + this.id;
-    canvas.mozOpaque = true;
     div.appendChild(canvas);
     this.canvas = canvas;
 
@@ -2142,6 +2141,7 @@ var PageView = function pageView(container, pdfPage, id, scale,
     }
 
     var ctx = canvas.getContext('2d');
+    ctx.clearRect(0, 0, canvas.width, canvas.height);
     // TODO(mack): use data attributes to store these
     ctx._scaleX = outputScale.sx;
     ctx._scaleY = outputScale.sy;
@@ -2363,7 +2363,6 @@ var ThumbnailView = function thumbnailView(container, pdfPage, id) {
   function getPageDrawContext() {
     var canvas = document.createElement('canvas');
     canvas.id = 'thumbnail' + id;
-    canvas.mozOpaque = true;
 
     canvas.width = canvasWidth;
     canvas.height = canvasHeight;