--- a/dom/media/Intervals.h
+++ b/dom/media/Intervals.h
@@ -396,17 +396,17 @@ public:
 
   SelfType operator+ (const SelfType& aIntervals) const
   {
     SelfType intervals(*this);
     intervals.Add(aIntervals);
     return intervals;
   }
 
-  SelfType operator+ (const ElemType& aInterval)
+  SelfType operator+ (const ElemType& aInterval) const
   {
     SelfType intervals(*this);
     intervals.Add(aInterval);
     return intervals;
   }
 
   friend SelfType operator+ (const ElemType& aInterval,
                              const SelfType& aIntervals)
@@ -437,17 +437,24 @@ public:
   SelfType& operator-= (const SelfType& aIntervals)
   {
     for (const auto& interval : aIntervals.mIntervals) {
       *this -= interval;
     }
     return *this;
   }
 
-  SelfType operator- (const ElemType& aInterval)
+  SelfType operator- (const SelfType& aInterval) const
+  {
+    SelfType intervals(*this);
+    intervals -= aInterval;
+    return intervals;
+  }
+
+  SelfType operator- (const ElemType& aInterval) const
   {
     SelfType intervals(*this);
     intervals -= aInterval;
     return intervals;
   }
 
   // Mutate this IntervalSet to be the union of this and aOther.
   SelfType& Union(const SelfType& aOther)