--- a/mfbt/Assertions.h
+++ b/mfbt/Assertions.h
@@ -359,16 +359,19 @@ MOZ_END_EXTERN_C
  * MOZ_RELEASE_ASSERT, which applies to non-debug builds as well.
  *
  * MOZ_DIAGNOSTIC_ASSERT works like MOZ_RELEASE_ASSERT in Nightly/Aurora and
  * MOZ_ASSERT in Beta/Release - use this when a condition is potentially rare
  * enough to require real user testing to hit, but is not security-sensitive.
  * This can cause user pain, so use it sparingly. If a MOZ_DIAGNOSTIC_ASSERT
  * is firing, it should promptly be converted to a MOZ_ASSERT while the failure
  * is being investigated, rather than letting users suffer.
+ *
+ * MOZ_DIAGNOSTIC_ASSERT_ENABLED is defined when MOZ_DIAGNOSTIC_ASSERT is like
+ * MOZ_RELEASE_ASSERT rather than MOZ_ASSERT.
  */
 
 /*
  * Implement MOZ_VALIDATE_ASSERT_CONDITION_TYPE, which is used to guard against
  * accidentally passing something unintended in lieu of an assertion condition.
  */
 
 #ifdef __cplusplus
@@ -441,18 +444,22 @@ struct AssertionConditionType
 #ifdef DEBUG
 #  define MOZ_ASSERT(...) MOZ_RELEASE_ASSERT(__VA_ARGS__)
 #else
 #  define MOZ_ASSERT(...) do { } while (0)
 #endif /* DEBUG */
 
 #ifdef RELEASE_OR_BETA
 #  define MOZ_DIAGNOSTIC_ASSERT MOZ_ASSERT
+#  ifdef DEBUG
+#    define MOZ_DIAGNOSTIC_ASSERT_ENABLED 1
+#  endif
 #else
 #  define MOZ_DIAGNOSTIC_ASSERT MOZ_RELEASE_ASSERT
+#  define MOZ_DIAGNOSTIC_ASSERT_ENABLED 1
 #endif
 
 /*
  * MOZ_ASSERT_IF(cond1, cond2) is equivalent to MOZ_ASSERT(cond2) if cond1 is
  * true.
  *
  *   MOZ_ASSERT_IF(isPrime(num), num == 2 || isOdd(num));
  *