diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index d341f1a94ac9..84be42da7a11 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -891,11 +891,13 @@ CalculateContainingBlockSizeForAbsolutes(const nsHTMLReflowState& aReflowState,
     // frame and not always sticking them in block frames.
 
     // First, find the reflow state for the outermost frame for this
-    // content.
+    // content, except for fieldsets where the inner anonymous frame has
+    // the correct padding area with the legend taken into account.
     const nsHTMLReflowState* aLastRS = &aReflowState;
     const nsHTMLReflowState* lastButOneRS = &aReflowState;
     while (aLastRS->parentReflowState &&
-           aLastRS->parentReflowState->frame->GetContent() == frame->GetContent()) {
+           aLastRS->parentReflowState->frame->GetContent() == frame->GetContent() &&
+           aLastRS->parentReflowState->frame->GetType() != nsGkAtoms::fieldSetFrame) {
       lastButOneRS = aLastRS;
       aLastRS = aLastRS->parentReflowState;
     }
diff --git a/layout/reftests/forms/fieldset/abs-pos-child-sizing-ref.html b/layout/reftests/forms/fieldset/abs-pos-child-sizing-ref.html
new file mode 100644
index 000000000000..405cfaf0a9d9
--- /dev/null
+++ b/layout/reftests/forms/fieldset/abs-pos-child-sizing-ref.html
@@ -0,0 +1,79 @@
+<!DOCTYPE HTML>
+<html><head>
+    <meta charset="utf-8">
+    <title>Testcase for bug 971653</title>
+    <style type="text/css">
+        html,body {
+            color:black; background-color:white; font-size:16px; padding:0; margin:0;
+        }
+
+div {
+    position: absolute;
+    background: rgba(0,255,0,0.5);
+    top:0; bottom:0;
+    left:0; right:0;
+    z-index: 0;
+}
+c {
+    display: block;
+    background: blue;
+    height:100%;
+}
+
+legend { height: 24px; }
+
+fieldset {
+    position: absolute;
+    border: 2px green solid;
+    width: 100px;
+    height: 100px;
+    margin-bottom:40px;
+}
+
+#t1,#t3 { padding: 3px 5px 7px 11px; }
+#t2,#t4 { padding: 0; }
+
+#t1 div, #t3 div {
+    /* same numbers as padding above */
+    left: -11px; right: -5px;
+    top:  -3px;  bottom:-7px;
+}
+
+#t2 { top: 120px; }
+#t3 { top: 240px; }
+#t4 { top: 360px; }
+
+x {
+    position: absolute;
+    width:100px;
+    height:100px;
+}
+#t1 x, #t2 x {
+    height:78px; /* compensate for legend */
+}
+    </style>
+</head>
+<body>
+
+
+<fieldset id="t1">
+    <legend>legend</legend>
+    <x><div></div><c></c></x>
+</fieldset>
+
+<fieldset id="t2">
+    <legend>legend</legend>
+    <x><div></div></x>
+</fieldset>
+
+<fieldset id="t3">
+    <x><div></div><c></c></x>
+</fieldset>
+
+<fieldset id="t4">
+    <x><div></div></x>
+</fieldset>
+
+
+</body>
+</html>
diff --git a/layout/reftests/forms/fieldset/abs-pos-child-sizing.html b/layout/reftests/forms/fieldset/abs-pos-child-sizing.html
new file mode 100644
index 000000000000..72d47deac9bd
--- /dev/null
+++ b/layout/reftests/forms/fieldset/abs-pos-child-sizing.html
@@ -0,0 +1,64 @@
+<!DOCTYPE HTML>
+<html><head>
+    <meta charset="utf-8">
+    <title>Testcase for bug 971653</title>
+    <style type="text/css">
+        html,body {
+            color:black; background-color:white; font-size:16px; padding:0; margin:0;
+        }
+
+div {
+    position: absolute;
+    background: rgba(0,255,0,0.5);
+    top:0; bottom:0;
+    left:0; right:0;
+    z-index: 0;
+}
+c {
+    display: block;
+    background: blue;
+    height:100%;
+}
+
+legend { height: 24px; }
+
+fieldset {
+    position: absolute;
+    border: 2px green solid;
+    width: 100px;
+    height: 100px;
+    margin-bottom:40px;
+}
+
+#t1,#t3 { padding: 3px 5px 7px 11px; }
+#t2,#t4 { padding: 0; }
+
+#t2 { top: 120px; }
+#t3 { top: 240px; }
+#t4 { top: 360px; }
+    </style>
+</head>
+<body>
+
+
+<fieldset id="t1">
+    <legend>legend</legend>
+    <div></div><c></c>
+</fieldset>
+
+<fieldset id="t2">
+    <legend>legend</legend>
+    <div></div>
+</fieldset>
+
+<fieldset id="t3">
+    <div></div><c></c>
+</fieldset>
+
+<fieldset id="t4">
+    <div></div>
+</fieldset>
+
+
+</body>
+</html>
diff --git a/layout/reftests/forms/fieldset/reftest.list b/layout/reftests/forms/fieldset/reftest.list
index da8e4f81c0e5..bd5e86e66bde 100644
--- a/layout/reftests/forms/fieldset/reftest.list
+++ b/layout/reftests/forms/fieldset/reftest.list
@@ -9,3 +9,4 @@ random-if(B2G) == fieldset-overflow-auto-1.html fieldset-overflow-auto-1-ref.htm
 == relpos-legend-1.html relpos-legend-1-ref.html
 == relpos-legend-2.html relpos-legend-2-ref.html
 test-pref(layout.css.sticky.enabled,true) == sticky-legend-1.html sticky-legend-1-ref.html
+== abs-pos-child-sizing.html abs-pos-child-sizing-ref.html
