--- a/layout/generic/nsFloatManager.cpp
+++ b/layout/generic/nsFloatManager.cpp
@@ -991,18 +991,24 @@ nsFloatManager::EllipseShapeInfo::LineEd
     bEndIsBelowOrAtCenter ? aBEnd : aBEnd + (mCenter.y - aBEnd) * 2 - 1);
 
   MOZ_ASSERT(bSmallestWithinIntervals >= mCenter.y &&
              bSmallestWithinIntervals < BEnd(),
              "We should have a block value within the intervals.");
 
   size_t index = MinIntervalIndexContainingY(mIntervals,
                                              bSmallestWithinIntervals);
-  MOZ_ASSERT(index < mIntervals.Length(),
-             "We should have found a matching interval for this block value.");
+  if (index >= mIntervals.Length()) {
+    // Shouldn't occur, but this indicates that this float area doesn't
+    // influence the outcome. We also avoid an illegal array index reference
+    // by returning early here.
+    NS_WARNING("We should have found a matching interval for this "
+               "block value.");
+    return 0;
+  }
 
   // The interval is storing the line right value. If aIsLineLeft is true,
   // return the line right value reflected about the center. Since this is
   // an inline measurement, it's just checking the distance to an edge, and
   // not a collision with a specific pixel. For that reason, we don't need
   // to subtract 1 from the reflection, as we did with the block reflection.
   nscoord iLineRight = mIntervals[index].XMost();
   return aIsLineLeft ? iLineRight - (iLineRight - mCenter.x) * 2