--- a/mfbt/Assertions.h
+++ b/mfbt/Assertions.h
@@ -5,16 +5,17 @@
 
 /* Implementations of runtime and static assertion macros for C and C++. */
 
 #ifndef mozilla_Assertions_h_
 #define mozilla_Assertions_h_
 
 #include "mozilla/Attributes.h"
 #include "mozilla/Compiler.h"
+#include "mozilla/Likely.h"
 
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #ifdef WIN32
    /*
     * TerminateProcess and GetCurrentProcess are defined in <winbase.h>, which
     * further depends on <windef.h>.  We hardcode these few definitions manually
@@ -243,25 +244,25 @@ MOZ_ReportAssertionFailure(const char* s
  *
  * MOZ_ASSERT has no effect in non-debug builds.  It is designed to catch bugs
  * *only* during debugging, not "in the field".
  */
 #ifdef DEBUG
    /* First the single-argument form. */
 #  define MOZ_ASSERT_HELPER1(expr) \
      do { \
-       if (!(expr)) { \
+       if (MOZ_UNLIKELY(!(expr))) { \
          MOZ_ReportAssertionFailure(#expr, __FILE__, __LINE__); \
          MOZ_CRASH(); \
        } \
      } while (0)
    /* Now the two-argument form. */
 #  define MOZ_ASSERT_HELPER2(expr, explain) \
      do { \
-       if (!(expr)) { \
+       if (MOZ_UNLIKELY(!(expr))) { \
          MOZ_ReportAssertionFailure(#expr " (" explain ")", __FILE__, __LINE__); \
          MOZ_CRASH(); \
        } \
      } while (0)
    /* And now, helper macrology up the wazoo. */
    /*
     * Count the number of arguments passed to MOZ_ASSERT, very carefully
     * tiptoeing around an MSVC bug where it improperly expands __VA_ARGS__ as a