diff --git a/js/src/jsapi-tests/testPrintf.cpp b/js/src/jsapi-tests/testPrintf.cpp
index 603a6a872529..332d0d85cdd9 100644
--- a/js/src/jsapi-tests/testPrintf.cpp
+++ b/js/src/jsapi-tests/testPrintf.cpp
@@ -25,7 +25,11 @@ static bool MOZ_FORMAT_PRINTF(2, 3)
   return output && !strcmp(output.get(), expect);
 }
 
-static const char* zero() { return nullptr; }
+static const char* zero() {
+  // gcc 9 is altogether too clever about detecting that this will always
+  // return nullptr. Do not replace 0x10 with 0x1; it will no longer work.
+  return uintptr_t(&zero) == 0x10 ? "never happens" : nullptr;
+}
 
 BEGIN_TEST(testPrintf) {
   CHECK(print_one("23", "%d", 23));
diff --git a/js/src/shell/OSObject.cpp b/js/src/shell/OSObject.cpp
index b4eb635ef581..fc52922aacbb 100644
--- a/js/src/shell/OSObject.cpp
+++ b/js/src/shell/OSObject.cpp
@@ -141,8 +141,8 @@ JSString* ResolvePath(JSContext* cx, HandleString filenameStr,
     // The docs say it can return EINVAL, but the compiler says it's void
     _splitpath(scriptFilename.get(), nullptr, buffer, nullptr, nullptr);
 #else
-    strncpy(buffer, scriptFilename.get(), PATH_MAX + 1);
-    if (buffer[PATH_MAX] != '\0') {
+    strncpy(buffer, scriptFilename.get(), PATH_MAX);
+    if (buffer[PATH_MAX - 1] != '\0') {
       return nullptr;
     }
 
diff --git a/mfbt/DefineEnum.h b/mfbt/DefineEnum.h
index d414a90d9bb6..afcff10e5212 100644
--- a/mfbt/DefineEnum.h
+++ b/mfbt/DefineEnum.h
@@ -90,8 +90,8 @@
 /*
  * A helper macro for asserting that an enumerator does not have an initializer.
  *
- * The static_assert and the comparison to 0 are just scaffolding; the
- * important part is forming the expression |aEnumName::aEnumeratorDecl|.
+ * The static_assert and the comparison are just scaffolding; the important
+ * part is forming the expression |aEnumName::aEnumeratorDecl|.
  *
  * If |aEnumeratorDecl| is just the enumerator name without an identifier,
  * this expression compiles fine. However, if |aEnumeratorDecl| includes an
@@ -99,19 +99,20 @@
  * compile in expression context, since |eEnumerator| is not an lvalue.
  *
  * (The static_assert itself should always pass in the absence of the above
- * error, since you can't get a negative enumerator value without having
- * an initializer somewhere. It just provides a place to put the expression
- * we want to form.)
+ * error, since turning on a bit can only increase an integer value. It just
+ * provides a place to put the expression we want to form.)
  */
+
 #define MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER(aEnumName, aEnumeratorDecl) \
   static_assert(                                                             \
-      (aEnumName::aEnumeratorDecl) >= aEnumName(0),                          \
+      int(aEnumName::aEnumeratorDecl) <=                                     \
+          (int(aEnumName::aEnumeratorDecl) | 1),                             \
       "MOZ_DEFINE_ENUM does not allow enumerators to have initializers");
 
 #define MOZ_DEFINE_ENUM_IMPL(aEnumName, aClassSpec, aBaseSpec, aEnumerators) \
   enum aClassSpec aEnumName aBaseSpec{MOZ_UNWRAP_ARGS aEnumerators};         \
   constexpr size_t k##aEnumName##Count = MOZ_ARG_COUNT aEnumerators;         \
-  constexpr aEnumName k##Highest##aEnumName =                                \
+  constexpr aEnumName kHighest##aEnumName =                                  \
       aEnumName(k##aEnumName##Count - 1);                                    \
   MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName, ),      \
                aEnumerators)
@@ -132,7 +133,7 @@
                                             aEnumerators)                     \
   enum aClassSpec aEnumName aBaseSpec{MOZ_UNWRAP_ARGS aEnumerators};          \
   constexpr static size_t s##aEnumName##Count = MOZ_ARG_COUNT aEnumerators;   \
-  constexpr static aEnumName s##Highest##aEnumName =                          \
+  constexpr static aEnumName sHighest##aEnumName =                            \
       aEnumName(s##aEnumName##Count - 1);                                     \
   MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName, ),       \
                aEnumerators)