diff --git a/mfbt/RollingMean.h b/mfbt/RollingMean.h
index 5add14c879f..8cc3148e960 100644
--- a/mfbt/RollingMean.h
+++ b/mfbt/RollingMean.h
@@ -86,7 +86,7 @@ public:
   T mean()
   {
     MOZ_ASSERT(!empty());
-    return T(mTotal / mValues.length());
+    return T(mTotal / int64_t(mValues.length()));
   }
 
   bool empty()
diff --git a/xpcom/ds/TimeStamp.h b/xpcom/ds/TimeStamp.h
index fdb87559d60..036ef578c30 100644
--- a/xpcom/ds/TimeStamp.h
+++ b/xpcom/ds/TimeStamp.h
@@ -181,6 +181,11 @@ private:
   // If required, use MultDouble explicitly and with care.
   BaseTimeDuration operator*(const double aMultiplier) const = delete;
 
+  // Block double divisor (for the same reason, and because dividing by
+  // fractional values would otherwise invoke the int64_t variant, and rounding
+  // the passed argument can then cause divide-by-zero) - Bug 1147491.
+  BaseTimeDuration operator/(const double aDivisor) const = delete;
+
 public:
   BaseTimeDuration MultDouble(double aMultiplier) const
   {