diff --git a/mozilla/layout/generic/nsBlockFrame.cpp b/mozilla/layout/generic/nsBlockFrame.cpp
index aad0e78818f..69597d0d22f 100644
--- a/mozilla/layout/generic/nsBlockFrame.cpp
+++ b/mozilla/layout/generic/nsBlockFrame.cpp
@@ -1918,7 +1918,7 @@ nsBlockFrame::ReflowDirtyLines(nsBlockReflowState& aState)
       // to the result of nextLine->ShouldApplyTopMargin.
       if (line.next() != end_lines()) {
         PRBool maybeWasEmpty = oldY == line.next()->mBounds.y;
-        PRBool isEmpty = line->mBounds.width == 0 && line->CachedIsEmpty();
+        PRBool isEmpty = line->CachedIsEmpty();
         if (maybeReflowingForFirstTime /*1*/ ||
             (isEmpty || maybeWasEmpty) /*2/3/4*/) {
           line.next()->MarkPreviousMarginDirty();
diff --git a/mozilla/layout/reftests/bugs/421239-1-ref.html b/mozilla/layout/reftests/bugs/421239-1-ref.html
new file mode 100644
index 00000000000..6e75fc08e6e
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/421239-1-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <style>
+    div#a { margin: 100px 0px 100px 0px;  }
+  </style>
+</head>
+<body>
+  <div id="a"></div>
+  <div>b</div>
+</body>
+</html>
diff --git a/mozilla/layout/reftests/bugs/421239-1.html b/mozilla/layout/reftests/bugs/421239-1.html
new file mode 100644
index 00000000000..d4d94b0bacc
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/421239-1.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<head>
+  <style>
+    div#a { margin: 100px 0px 100px 0px;  }
+  </style>
+  <script>
+  function boom()
+  {
+    var a = document.getElementById("a");
+    a.removeChild(a.firstChild);
+    document.documentElement.className = "";
+  }
+  </script>
+</head>
+<body onload="boom();">
+  <div id="a">a</div>
+  <div>b</div>
+</body>
+</html>
diff --git a/mozilla/layout/reftests/bugs/421239-2-ref.html b/mozilla/layout/reftests/bugs/421239-2-ref.html
new file mode 100644
index 00000000000..3d2770879a8
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/421239-2-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <style>
+    div#a { margin: 0px 0px 100px 0px;  }
+  </style>
+</head>
+<body>
+  <div id="a"></div>
+  <div>b</div>
+</body>
+</html>
diff --git a/mozilla/layout/reftests/bugs/421239-2.html b/mozilla/layout/reftests/bugs/421239-2.html
new file mode 100644
index 00000000000..39f5bf47e4c
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/421239-2.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<head>
+  <style>
+    div#a { margin: 0px 0px 100px 0px;  }
+  </style>
+  <script>
+  function boom()
+  {
+    var a = document.getElementById("a");
+    a.removeChild(a.firstChild);
+    document.documentElement.className = "";
+  }
+  </script>
+</head>
+<body onload="boom();">
+  <div id="a">a</div>
+  <div>b</div>
+</body>
+</html>
