diff --git a/content/base/test/test_fileapi_slice.html b/content/base/test/test_fileapi_slice.html
index bf97307eafc9..2b5594ce93e1 100644
--- a/content/base/test/test_fileapi_slice.html
+++ b/content/base/test/test_fileapi_slice.html
@@ -13,8 +13,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=575946
 <body>
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=575946">Mozilla Bug 575946</a>
 <p id="display">
-  <canvas id=canvas width=1000 height=1000 hidden></canvas>
-  <canvas id=testcanvas hidden></canvas>
+  <canvas id=canvas width=1100 height=1100 hidden moz-opaque></canvas>
+  <canvas id=testcanvas hidden moz-opaque></canvas>
   <input id="fileList" type="file"></input>
 </p>
 <div id="content" style="display: none">
diff --git a/dom/tests/mochitest/bugs/test_bug389366.html b/dom/tests/mochitest/bugs/test_bug389366.html
index e4d4c1f04668..01d77a5d7048 100644
--- a/dom/tests/mochitest/bugs/test_bug389366.html
+++ b/dom/tests/mochitest/bugs/test_bug389366.html
@@ -31,7 +31,7 @@ function test()
   var canvas1 = document.getElementById("canvas1");
   var ctx1 = canvas1.getContext("2d");
 
-  ctx1.globalAlpha = 0.5;
+  ctx1.globalAlpha = 0.502;
   ctx1.fillStyle = "#FF0000";
   ctx1.fillRect(0, 0, 10, 10);
 
