--- a/dom/smil/nsSMILKeySpline.cpp
+++ b/dom/smil/nsSMILKeySpline.cpp
@@ -71,16 +71,20 @@ nsSMILKeySpline::GetSlope(double aT,
                           double aA2)
 {
   return 3.0 * A(aA1, aA2)*aT*aT + 2.0 * B(aA1, aA2) * aT + C(aA1);
 }
 
 double
 nsSMILKeySpline::GetTForX(double aX) const
 {
+  // Early return when aX == 1.0 to avoid floating-point inaccuracies.
+  if (aX == 1.0) {
+    return 1.0;
+  }
   // Find interval where t lies
   double intervalStart = 0.0;
   const double* currentSample = &mSampleValues[1];
   const double* const lastSample = &mSampleValues[kSplineTableSize - 1];
   for (; currentSample != lastSample && *currentSample <= aX;
         ++currentSample) {
     intervalStart += kSampleStepSize;
   }