diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index e2676fb7bd6d..fe8947f60975 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -829,11 +829,11 @@ void ContentChild::SetProcessName(const nsACString& aName,
                                   const nsACString* aCurrentProfile) {
   char* name;
   if ((name = PR_GetEnv("MOZ_DEBUG_APP_PROCESS")) && aName.EqualsASCII(name)) {
-#ifdef OS_POSIX
+#ifdef XP_UNIX
     printf_stderr("\n\nCHILDCHILDCHILDCHILD\n  [%s] debug me @%d\n\n", name,
                   getpid());
     sleep(30);
-#elif defined(OS_WIN)
+#elif defined(XP_WIN)
     // Windows has a decent JIT debugging story, so NS_DebugBreak does the
     // right thing.
     NS_DebugBreak(NS_DEBUG_BREAK,
@@ -1735,11 +1735,11 @@ mozilla::ipc::IPCResult ContentChild::RecvSetProcessSandbox(
 
   CrashReporter::AnnotateCrashReport(
       CrashReporter::Annotation::ContentSandboxEnabled, sandboxEnabled);
-#  if defined(XP_LINUX) && !defined(OS_ANDROID)
+#  if defined(XP_LINUX) && !defined(ANDROID)
   CrashReporter::AnnotateCrashReport(
       CrashReporter::Annotation::ContentSandboxCapabilities,
       static_cast<int>(SandboxInfo::Get().AsInteger()));
-#  endif /* XP_LINUX && !OS_ANDROID */
+#  endif /* XP_LINUX && !ANDROID */
 #endif   /* MOZ_SANDBOX */
 
   return IPC_OK();
diff --git a/dom/ipc/ContentProcess.cpp b/dom/ipc/ContentProcess.cpp
index 8761b258602b..54db9a7d9044 100644
--- a/dom/ipc/ContentProcess.cpp
+++ b/dom/ipc/ContentProcess.cpp
@@ -31,7 +31,7 @@ static nsresult GetGREDir(nsIFile** aResult) {
   nsresult rv = XRE_GetBinaryPath(getter_AddRefs(current));
   NS_ENSURE_SUCCESS(rv, rv);
 
-#ifdef OS_MACOSX
+#ifdef XP_DARWIN
   // Walk out of [subprocess].app/Contents/MacOS to the real GRE dir
   const int depth = 4;
 #else
@@ -47,7 +47,7 @@ static nsresult GetGREDir(nsIFile** aResult) {
     NS_ENSURE_TRUE(current, NS_ERROR_UNEXPECTED);
   }
 
-#ifdef OS_MACOSX
+#ifdef XP_DARWIN
   rv = current->SetNativeLeafName("Resources"_ns);
   NS_ENSURE_SUCCESS(rv, rv);
 #endif
diff --git a/dom/media/gmp/GMPChild.cpp b/dom/media/gmp/GMPChild.cpp
index 0d485345bc6d..99039998f68f 100644
--- a/dom/media/gmp/GMPChild.cpp
+++ b/dom/media/gmp/GMPChild.cpp
@@ -226,7 +226,7 @@ bool GMPChild::GetUTF8LibPath(nsACString& aOutLibPath) {
 
 #if defined(XP_MACOSX)
   nsAutoString binaryName = u"lib"_ns + baseName + u".dylib"_ns;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   nsAutoString binaryName = u"lib"_ns + baseName + u".so"_ns;
 #elif defined(XP_WIN)
   nsAutoString binaryName = baseName + u".dll"_ns;
diff --git a/dom/media/gmp/GMPProcessChild.cpp b/dom/media/gmp/GMPProcessChild.cpp
index 17c70079cb4a..2d673f27ba6e 100644
--- a/dom/media/gmp/GMPProcessChild.cpp
+++ b/dom/media/gmp/GMPProcessChild.cpp
@@ -19,14 +19,14 @@ GMPProcessChild::~GMPProcessChild() = default;
 bool GMPProcessChild::Init(int aArgc, char* aArgv[]) {
   nsAutoString pluginFilename;
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // NB: need to be very careful in ensuring that the first arg
   // (after the binary name) here is indeed the plugin module path.
   // Keep in sync with dom/plugins/PluginModuleParent.
   std::vector<std::string> values = CommandLine::ForCurrentProcess()->argv();
   MOZ_ASSERT(values.size() >= 2, "not enough args");
   CopyUTF8toUTF16(nsDependentCString(values[1].c_str()), pluginFilename);
-#elif defined(OS_WIN)
+#elif defined(XP_WIN)
   std::vector<std::wstring> values =
       CommandLine::ForCurrentProcess()->GetLooseValues();
   MOZ_ASSERT(values.size() >= 1, "not enough loose args");
diff --git a/dom/media/ipc/RDDProcessImpl.cpp b/dom/media/ipc/RDDProcessImpl.cpp
index 6ddd4ffe95b9..c49709e07275 100644
--- a/dom/media/ipc/RDDProcessImpl.cpp
+++ b/dom/media/ipc/RDDProcessImpl.cpp
@@ -8,7 +8,7 @@
 #include "mozilla/ipc/IOThreadChild.h"
 #include "mozilla/GeckoArgs.h"
 
-#if defined(OS_WIN) && defined(MOZ_SANDBOX)
+#if defined(XP_WIN) && defined(MOZ_SANDBOX)
 #  include "mozilla/sandboxTarget.h"
 #elif defined(__OpenBSD__) && defined(MOZ_SANDBOX)
 #  include "mozilla/SandboxSettings.h"
@@ -22,7 +22,7 @@ using namespace ipc;
 RDDProcessImpl::~RDDProcessImpl() = default;
 
 bool RDDProcessImpl::Init(int aArgc, char* aArgv[]) {
-#if defined(MOZ_SANDBOX) && defined(OS_WIN)
+#if defined(MOZ_SANDBOX) && defined(XP_WIN)
   // Preload AV dlls so we can enable Binary Signature Policy
   // to restrict further dll loads.
   LoadLibraryW(L"mozavcodec.dll");
diff --git a/dom/webauthn/PublicKeyCredential.cpp b/dom/webauthn/PublicKeyCredential.cpp
index c81f859a53a9..7001a55184ff 100644
--- a/dom/webauthn/PublicKeyCredential.cpp
+++ b/dom/webauthn/PublicKeyCredential.cpp
@@ -12,7 +12,7 @@
 #include "mozilla/HoldDropJSObjects.h"
 #include "mozilla/Preferences.h"
 
-#ifdef OS_WIN
+#ifdef XP_WIN
 #  include "WinWebAuthnManager.h"
 #endif
 
@@ -93,7 +93,7 @@ PublicKeyCredential::IsUserVerifyingPlatformAuthenticatorAvailable(
 //
 // If on latest windows, call system APIs, otherwise return false, as we don't
 // have other UVPAAs available at this time.
-#ifdef OS_WIN
+#ifdef XP_WIN
 
   if (WinWebAuthnManager::IsUserVerifyingPlatformAuthenticatorAvailable()) {
     promise->MaybeResolve(true);
@@ -129,7 +129,7 @@ PublicKeyCredential::IsExternalCTAP2SecurityKeySupported(GlobalObject& aGlobal,
     return nullptr;
   }
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     promise->MaybeResolve(true);
   } else {
diff --git a/dom/webauthn/WebAuthnManager.cpp b/dom/webauthn/WebAuthnManager.cpp
index 9a15f62bb447..215a5e69b39d 100644
--- a/dom/webauthn/WebAuthnManager.cpp
+++ b/dom/webauthn/WebAuthnManager.cpp
@@ -23,7 +23,7 @@
 #include "mozilla/ipc/BackgroundChild.h"
 #include "mozilla/ipc/PBackgroundChild.h"
 
-#ifdef OS_WIN
+#ifdef XP_WIN
 #  include "WinWebAuthnManager.h"
 #endif
 
@@ -451,7 +451,7 @@ already_AddRefed<Promise> WebAuthnManager::MakeCredential(
       adjustedTimeout, excludeList, rpInfo, userInfo, coseAlgos, extensions,
       authSelection, attestation, context->Top()->Id());
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (!WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     ListenForVisibilityEvents();
   }
@@ -658,7 +658,7 @@ already_AddRefed<Promise> WebAuthnManager::GetAssertion(
                                 extensions, aOptions.mUserVerification,
                                 context->Top()->Id());
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (!WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     ListenForVisibilityEvents();
   }
diff --git a/dom/webauthn/WebAuthnTransactionParent.cpp b/dom/webauthn/WebAuthnTransactionParent.cpp
index d73542d47884..48931846388f 100644
--- a/dom/webauthn/WebAuthnTransactionParent.cpp
+++ b/dom/webauthn/WebAuthnTransactionParent.cpp
@@ -13,7 +13,7 @@
 
 #include "nsThreadUtils.h"
 
-#ifdef OS_WIN
+#ifdef XP_WIN
 #  include "WinWebAuthnManager.h"
 #endif
 
@@ -24,7 +24,7 @@ mozilla::ipc::IPCResult WebAuthnTransactionParent::RecvRequestRegister(
     const WebAuthnMakeCredentialInfo& aTransactionInfo) {
   ::mozilla::ipc::AssertIsOnBackgroundThread();
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     WinWebAuthnManager* mgr = WinWebAuthnManager::Get();
     mgr->Register(this, aTransactionId, aTransactionInfo);
@@ -51,7 +51,7 @@ mozilla::ipc::IPCResult WebAuthnTransactionParent::RecvRequestSign(
     const WebAuthnGetAssertionInfo& aTransactionInfo) {
   ::mozilla::ipc::AssertIsOnBackgroundThread();
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     WinWebAuthnManager* mgr = WinWebAuthnManager::Get();
     mgr->Sign(this, aTransactionId, aTransactionInfo);
@@ -77,7 +77,7 @@ mozilla::ipc::IPCResult WebAuthnTransactionParent::RecvRequestCancel(
     const Tainted<uint64_t>& aTransactionId) {
   ::mozilla::ipc::AssertIsOnBackgroundThread();
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     WinWebAuthnManager* mgr = WinWebAuthnManager::Get();
     mgr->Cancel(this, aTransactionId);
@@ -124,7 +124,7 @@ void WebAuthnTransactionParent::ActorDestroy(ActorDestroyReason aWhy) {
   // Called either by Send__delete__() in RecvDestroyMe() above, or when
   // the channel disconnects. Ensure the token manager forgets about us.
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   if (WinWebAuthnManager::AreWebAuthNApisAvailable()) {
     WinWebAuthnManager* mgr = WinWebAuthnManager::Get();
     if (mgr) {
diff --git a/gfx/ipc/GPUProcessImpl.cpp b/gfx/ipc/GPUProcessImpl.cpp
index 00a5335ab6b5..3ea86843e0f5 100644
--- a/gfx/ipc/GPUProcessImpl.cpp
+++ b/gfx/ipc/GPUProcessImpl.cpp
@@ -9,7 +9,7 @@
 #include "mozilla/ipc/ProcessUtils.h"
 #include "mozilla/GeckoArgs.h"
 
-#if defined(OS_WIN) && defined(MOZ_SANDBOX)
+#if defined(XP_WIN) && defined(MOZ_SANDBOX)
 #  include "mozilla/sandboxTarget.h"
 #elif defined(__OpenBSD__) && defined(MOZ_SANDBOX)
 #  include "mozilla/SandboxSettings.h"
@@ -23,7 +23,7 @@ using namespace ipc;
 GPUProcessImpl::~GPUProcessImpl() = default;
 
 bool GPUProcessImpl::Init(int aArgc, char* aArgv[]) {
-#if defined(MOZ_SANDBOX) && defined(OS_WIN)
+#if defined(MOZ_SANDBOX) && defined(XP_WIN)
   mozilla::SandboxTarget::Instance()->StartSandbox();
 #elif defined(__OpenBSD__) && defined(MOZ_SANDBOX)
   StartOpenBSDSandbox(GeckoProcessType_GPU);
diff --git a/gfx/layers/ipc/ISurfaceAllocator.cpp b/gfx/layers/ipc/ISurfaceAllocator.cpp
index 7baca7a9b667..46c8c6703a07 100644
--- a/gfx/layers/ipc/ISurfaceAllocator.cpp
+++ b/gfx/layers/ipc/ISurfaceAllocator.cpp
@@ -26,7 +26,7 @@ void HostIPCAllocator::SendPendingAsyncMessages() {
   // Some type of AsyncParentMessageData message could have
   // one file descriptor (e.g. OpDeliverFence).
   // A number of file descriptors per gecko ipc message have a limitation
-  // on OS_POSIX (MACOSX or LINUX).
+  // on XP_UNIX (MACOSX or LINUX).
   static const uint32_t kMaxMessageNumber =
       IPC::Message::MAX_DESCRIPTORS_PER_MESSAGE;
 
diff --git a/gfx/layers/ipc/ImageBridgeParent.cpp b/gfx/layers/ipc/ImageBridgeParent.cpp
index 4d20d763cec2..ee4d0ccc617e 100644
--- a/gfx/layers/ipc/ImageBridgeParent.cpp
+++ b/gfx/layers/ipc/ImageBridgeParent.cpp
@@ -35,7 +35,7 @@
 #include "mozilla/layers/TextureHost.h"
 #include "nsThreadUtils.h"
 
-#if defined(OS_WIN)
+#if defined(XP_WIN)
 #  include "mozilla/layers/TextureD3D11.h"
 #endif
 
diff --git a/layout/build/nsLayoutStatics.cpp b/layout/build/nsLayoutStatics.cpp
index 1b6f45818cdb..f44cacb558c9 100644
--- a/layout/build/nsLayoutStatics.cpp
+++ b/layout/build/nsLayoutStatics.cpp
@@ -107,7 +107,7 @@
 #include "mozilla/dom/WebIDLGlobalNameHash.h"
 #include "mozilla/dom/U2FTokenManager.h"
 #include "mozilla/dom/WebAuthnController.h"
-#ifdef OS_WIN
+#ifdef XP_WIN
 #  include "mozilla/dom/WinWebAuthnManager.h"
 #endif
 #include "mozilla/dom/PointerEventHandler.h"
@@ -265,7 +265,7 @@ nsresult nsLayoutStatics::Initialize() {
   mozilla::dom::U2FTokenManager::Initialize();
   mozilla::dom::WebAuthnController::Initialize();
 
-#ifdef OS_WIN
+#ifdef XP_WIN
   mozilla::dom::WinWebAuthnManager::Initialize();
 #endif
 
diff --git a/netwerk/ipc/SocketProcessImpl.cpp b/netwerk/ipc/SocketProcessImpl.cpp
index 40c64dc2bb73..a863d4a8759b 100644
--- a/netwerk/ipc/SocketProcessImpl.cpp
+++ b/netwerk/ipc/SocketProcessImpl.cpp
@@ -14,14 +14,14 @@
 #include "mozilla/ipc/ProcessUtils.h"
 #include "mozilla/ipc/IOThreadChild.h"
 
-#if defined(OS_WIN) && defined(MOZ_SANDBOX)
+#if defined(XP_WIN) && defined(MOZ_SANDBOX)
 #  include "mozilla/sandboxTarget.h"
 #elif defined(__OpenBSD__) && defined(MOZ_SANDBOX)
 #  include "mozilla/SandboxSettings.h"
 #  include "prlink.h"
 #endif
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 #  include <unistd.h>  // For sleep().
 #endif
 
@@ -35,14 +35,14 @@ LazyLogModule gSocketProcessLog("socketprocess");
 SocketProcessImpl::~SocketProcessImpl() = default;
 
 bool SocketProcessImpl::Init(int aArgc, char* aArgv[]) {
-#ifdef OS_POSIX
+#ifdef XP_UNIX
   if (PR_GetEnv("MOZ_DEBUG_SOCKET_PROCESS")) {
     printf_stderr("\n\nSOCKETPROCESSnSOCKETPROCESS\n  debug me @ %d\n\n",
                   base::GetCurrentProcId());
     sleep(30);
   }
 #endif
-#if defined(MOZ_SANDBOX) && defined(OS_WIN)
+#if defined(MOZ_SANDBOX) && defined(XP_WIN)
   LoadLibraryW(L"nss3.dll");
   LoadLibraryW(L"softokn3.dll");
   LoadLibraryW(L"freebl3.dll");
diff --git a/toolkit/crashreporter/google-breakpad/src/processor/proc_maps_linux.cc b/toolkit/crashreporter/google-breakpad/src/processor/proc_maps_linux.cc
index 3c0dea25d218..a6f8953bc309 100644
--- a/toolkit/crashreporter/google-breakpad/src/processor/proc_maps_linux.cc
+++ b/toolkit/crashreporter/google-breakpad/src/processor/proc_maps_linux.cc
@@ -15,7 +15,7 @@
 #include "common/using_std_string.h"
 #include "processor/logging.h"
 
-#if defined(OS_ANDROID) && !defined(__LP64__)
+#if defined(ANDROID) && !defined(__LP64__)
 // In 32-bit mode, Bionic's inttypes.h defines PRI/SCNxPTR as an
 // unsigned long int, which is incompatible with Bionic's stdint.h
 // defining uintptr_t as an unsigned int:
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index 9158d4f9b8ad..d74c0d3e6314 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -5925,7 +5925,7 @@ int XRE_main(int argc, char* argv[], const BootstrapConfig& aConfig) {
 nsresult XRE_InitCommandLine(int aArgc, char* aArgv[]) {
   nsresult rv = NS_OK;
 
-#if defined(OS_WIN)
+#if defined(XP_WIN)
   CommandLine::Init(aArgc, aArgv);
 #else
 
diff --git a/toolkit/xre/nsEmbedFunctions.cpp b/toolkit/xre/nsEmbedFunctions.cpp
index fa236e0f8623..aaafe2b8c63a 100644
--- a/toolkit/xre/nsEmbedFunctions.cpp
+++ b/toolkit/xre/nsEmbedFunctions.cpp
@@ -226,15 +226,15 @@ int GetDebugChildPauseTime() {
   if (pauseStr && *pauseStr) {
     int pause = atoi(pauseStr);
     if (pause != 1) {  // must be !=1 since =1 enables the default pause time
-#if defined(OS_WIN)
+#if defined(XP_WIN)
       pause *= 1000;  // convert to ms
 #endif
       return pause;
     }
   }
-#ifdef OS_POSIX
+#ifdef XP_UNIX
   return 30;  // seconds
-#elif defined(OS_WIN)
+#elif defined(XP_WIN)
   return 10000;  // milliseconds
 #else
   return 0;
@@ -432,7 +432,7 @@ nsresult XRE_InitChildProcess(int aArgc, char* aArgv[],
   g_set_prgname(aArgv[0]);
 #endif
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
   if (PR_GetEnv("MOZ_DEBUG_CHILD_PROCESS") ||
       PR_GetEnv("MOZ_DEBUG_CHILD_PAUSE")) {
 #  if defined(XP_LINUX) && defined(DEBUG)
@@ -445,7 +445,7 @@ nsresult XRE_InitChildProcess(int aArgc, char* aArgv[],
         XRE_GetProcessTypeString(), base::GetCurrentProcId());
     sleep(GetDebugChildPauseTime());
   }
-#elif defined(OS_WIN)
+#elif defined(XP_WIN)
   if (PR_GetEnv("MOZ_DEBUG_CHILD_PROCESS")) {
     NS_DebugBreak(NS_DEBUG_BREAK,
                   "Invoking NS_DebugBreak() to debug child process", nullptr,
diff --git a/widget/InputData.cpp b/widget/InputData.cpp
index 0265aab5a815..fdff81821e2b 100644
--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -608,7 +608,7 @@ WidgetWheelEvent PinchGestureInput::ToWidgetEvent(nsIWidget* aWidget) const {
 }
 
 double PinchGestureInput::ComputeDeltaY(nsIWidget* aWidget) const {
-#if defined(OS_MACOSX)
+#if defined(XP_DARWIN)
   // This converts the pinch gesture value to a fake wheel event that has the
   // control key pressed so that pages can implement custom pinch gesture
   // handling. It may seem strange that this doesn't use a wheel event with
diff --git a/xpcom/base/Logging.cpp b/xpcom/base/Logging.cpp
index 44f74686a90b..fa1527346561 100644
--- a/xpcom/base/Logging.cpp
+++ b/xpcom/base/Logging.cpp
@@ -199,7 +199,7 @@ bool LimitFileToLessThanSize(const char* aFilename, uint32_t aSize,
 
     ScopedCloseFile temp;
 
-#if defined(OS_WIN)
+#if defined(XP_WIN)
     // This approach was cribbed from
     // https://searchfox.org/mozilla-central/rev/868935867c6241e1302e64cf9be8f56db0fd0d1c/xpcom/build/LateWriteChecks.cpp#158.
     HANDLE hFile;
@@ -222,7 +222,7 @@ bool LimitFileToLessThanSize(const char* aFilename, uint32_t aSize,
     }
 
     temp.reset(_fdopen(fd, "ab"));
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 
     // Coverity would prefer us to set a secure umask before using `mkstemp`.
     // However, the umask is process-wide, so setting it may lead to difficult
@@ -273,13 +273,13 @@ bool LimitFileToLessThanSize(const char* aFilename, uint32_t aSize,
     return false;
   }
 
-#if defined(OS_WIN)
+#if defined(XP_WIN)
   if (!::ReplaceFileA(aFilename, tempFilename, nullptr, 0, 0, 0)) {
     NS_WARNING(
         nsPrintfCString("ReplaceFileA failed: %lu\n", GetLastError()).get());
     return false;
   }
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   if (rename(tempFilename, aFilename)) {
     NS_WARNING(
         nsPrintfCString("rename failed: %s (%d)\n", strerror(errno), errno)
diff --git a/xpcom/build/XPCOMInit.cpp b/xpcom/build/XPCOMInit.cpp
index cd6c7f705289..96d6b44c1f2b 100644
--- a/xpcom/build/XPCOMInit.cpp
+++ b/xpcom/build/XPCOMInit.cpp
@@ -359,7 +359,7 @@ NS_InitXPCOM(nsIServiceManager** aResult, nsIFile* aBinDirectory,
   }
 
   if ((sCommandLineWasInitialized = !CommandLine::IsInitialized())) {
-#ifdef OS_WIN
+#ifdef XP_WIN
     CommandLine::Init(0, nullptr);
 #else
     nsCOMPtr<nsIFile> binaryFile;