diff --git a/layout/style/test/flexbox_layout_testcases.js b/layout/style/test/flexbox_layout_testcases.js
index d6295bd685e8..9230166313bb 100644
--- a/layout/style/test/flexbox_layout_testcases.js
+++ b/layout/style/test/flexbox_layout_testcases.js
@@ -12,12 +12,17 @@
  * hash-entry for each CSS property that is to be set.  In these per-property
  * entries, the key is the property-name, and the value can be either of the
  * following:
- *  (a) the property's specified value
- *  ...or...
- *  (b) an array with 2 entries: [specifiedValue, expectedComputedValue] if the
- *      property's computed value is intended to be checked. The first entry
- *      (for the specified value) may be null; this means that no value should
- *      be explicitly specified for this property.
+ *  (a) the property's specified value (which indicates that we don't need to
+ *      bother checking the computed value of this particular property)
+ *  ...OR...
+ *  (b) an array with 2-3 entries...
+ *        [specifiedValue, expectedComputedValue (, epsilon) ]
+ *      ...which indicates that the property's computed value should be
+ *      checked.  The array's first entry (for the specified value) may be
+ *      null; this means that no value should be explicitly specified for this
+ *      property. The second entry is the property's expected computed
+ *      value. The third (optional) entry is an epsilon value, which allows for
+ *      fuzzy equality when testing the computed value.
  *
  * To allow these testcases to be re-used in both horizontal and vertical
  * flex containers, we specify "width"/"min-width"/etc. using the aliases
@@ -385,7 +390,7 @@ var gFlexboxTestcases =
        },
        {
          "flex": "1",
-         "_main-size": [ null,  "1px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
      ]
  },
@@ -416,15 +421,15 @@ var gFlexboxTestcases =
        },
        {
          "flex": "1",
-         "_main-size": [ null,  "1px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
        {
          "flex": "1",
-         "_main-size": [ null,  "1px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
        {
          "flex": "1",
-         "_main-size": [ null,  "1px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
      ]
  },
@@ -441,19 +446,19 @@ var gFlexboxTestcases =
          "flex": "1",
          // NOTE: Expected value is off slightly, from float error when
          // resolving flexible lengths:
-         "_main-size": [ null,  "0.966667px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
        {
          "flex": "1",
          // NOTE: Expected value is off slightly, from float error when
          // resolving flexible lengths:
-         "_main-size": [ null,  "0.983333px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
        {
          "flex": "1",
          // NOTE: Expected value is off slightly, from float error when
          // resolving flexible lengths:
-         "_main-size": [ null,  "0.983333px" ]
+         "_main-size": [ null,  "1px", 0.2 ]
        },
        {
          "flex": "3000000",
diff --git a/layout/style/test/test_flexbox_layout.html b/layout/style/test/test_flexbox_layout.html
index 5712d4a220db..c484030be676 100644
--- a/layout/style/test/test_flexbox_layout.html
+++ b/layout/style/test/test_flexbox_layout.html
@@ -52,6 +52,19 @@ function setPossiblyAliasedProperty(aElem, aPropertyName, aPropertyValue,
   }
 }
 
+// Helper function to strip "px" off the end of a string
+// (so that we can compare two lengths using "isfuzzy()" with an epsilon)
+function stripPx(aLengthInPx)
+{
+  let pxOffset = aLengthInPx.length - 2; // subtract off length of "px"
+
+  // Sanity-check the arg:
+  ok(pxOffset > 0 && aLengthInPx.substr(pxOffset) == "px",
+     "expecting value with 'px' units");
+
+  return aLengthInPx.substr(0, pxOffset);
+}
+
 // The main test function.
 // aFlexboxTestcase is an entry from the list in flexbox_layout_testcases.js
 function testFlexboxTestcase(aFlexboxTestcase, aFlexDirection, aPropertyMapping)
@@ -90,7 +103,8 @@ function testFlexboxTestcase(aFlexboxTestcase, aFlexDirection, aPropertyMapping)
 
       // SANITY CHECK:
       if (Array.isArray(aChildSpec[propName])) {
-        is(aChildSpec[propName].length, 2,
+        ok(aChildSpec[propName].length >= 2 &&
+           aChildSpec[propName].length <= 3,
            "unexpected number of elements in array within child spec");
       }
 
@@ -120,8 +134,20 @@ function testFlexboxTestcase(aFlexboxTestcase, aFlexDirection, aPropertyMapping)
         let expectedVal = childSpec[propName][1];
         let actualPropName = (propName in aPropertyMapping ?
                               aPropertyMapping[propName] : propName);
-        is(getComputedStyleWrapper(child, actualPropName), expectedVal,
-           "computed value of '" + actualPropName + "' should match expected");
+        let actualVal = getComputedStyleWrapper(child, actualPropName);
+        let message = "computed value of '" + actualPropName +
+                      "' should match expected";
+
+        if (childSpec[propName].length > 2) {
+          // 3rd entry in array is epsilon
+          // Need to strip off "px" units in order to use epsilon:
+          let actualValNoPx = stripPx(actualVal);
+          let expectedValNoPx = stripPx(expectedVal);
+          isfuzzy(actualValNoPx, expectedValNoPx,
+                  childSpec[propName][2], message);
+        } else {
+          is(actualVal, expectedVal, message);
+        }
       }
     }