diff --git a/build/clang-plugin/tests/moz.build b/build/clang-plugin/tests/moz.build
index b4fd9e0b7e97..4e5d427c4e92 100644
--- a/build/clang-plugin/tests/moz.build
+++ b/build/clang-plugin/tests/moz.build
@@ -94,8 +94,6 @@ COMPILE_FLAGS["OS_CFLAGS"] = [
     "-Xclang",
     "-verify",
     "-ferror-limit=0",
-    "-Xclang",
-    "-std=c11",
     "-Wno-invalid-noreturn",
 ]
 
diff --git a/media/ffvpx/libavcodec/moz.build b/media/ffvpx/libavcodec/moz.build
index 6f09049a6068..4fc5b9193edd 100644
--- a/media/ffvpx/libavcodec/moz.build
+++ b/media/ffvpx/libavcodec/moz.build
@@ -140,11 +140,6 @@ LOCAL_INCLUDES += [
     '/media/libvorbis',
 ]
 
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-CFLAGS += c11_flags
-
 if not CONFIG["MOZ_SYSTEM_LIBVPX"]:
   LOCAL_INCLUDES += ['/media/libvpx']
 else:
diff --git a/media/ffvpx/libavutil/moz.build b/media/ffvpx/libavutil/moz.build
index 0d40b19c8e0b..4d812ac57767 100644
--- a/media/ffvpx/libavutil/moz.build
+++ b/media/ffvpx/libavutil/moz.build
@@ -78,11 +78,6 @@ EXPORTS.ffvpx = [
     "tx.h"
 ]
 
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-CFLAGS += c11_flags
-
 SYMBOLS_FILE =  'avutil.symbols'
 NoVisibilityFlags()
 
diff --git a/modules/freetype2/moz.build b/modules/freetype2/moz.build
index 188c60c3402e..8c7124a24cda 100644
--- a/modules/freetype2/moz.build
+++ b/modules/freetype2/moz.build
@@ -6,8 +6,6 @@
 
 DEFINES['FT2_BUILD_LIBRARY'] = True
 
-CFLAGS += ['-std=c99']
-
 LOCAL_INCLUDES += [
     'include',
 ]
diff --git a/modules/libmar/sign/moz.build b/modules/libmar/sign/moz.build
index 92f0d6cefd33..82f9623b713d 100644
--- a/modules/libmar/sign/moz.build
+++ b/modules/libmar/sign/moz.build
@@ -22,9 +22,3 @@ DEFINES["MAR_NSS"] = True
 
 if CONFIG["OS_ARCH"] == "WINNT":
     USE_STATIC_LIBS = True
-
-# C11 for static_assert
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-CFLAGS += c11_flags
diff --git a/modules/libmar/src/moz.build b/modules/libmar/src/moz.build
index 5c40291e92b4..ee72adb3ad93 100644
--- a/modules/libmar/src/moz.build
+++ b/modules/libmar/src/moz.build
@@ -16,12 +16,6 @@ HOST_SOURCES += [
 ]
 HostLibrary("hostmar")
 
-# C11 for static_assert
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-HOST_CFLAGS += c11_flags
-
 LOCAL_INCLUDES += [
     "../../../other-licenses/nsis/Contrib/CityHash/cityhash",
 ]
@@ -31,8 +25,6 @@ if CONFIG["MOZ_BUILD_APP"] != "tools/update-packaging":
 
     UNIFIED_SOURCES += HOST_SOURCES
 
-    CFLAGS += c11_flags
-
     FORCE_STATIC_LIB = True
 
     if CONFIG["OS_ARCH"] == "WINNT":
diff --git a/modules/libmar/tool/moz.build b/modules/libmar/tool/moz.build
index b2772544d50d..16e937889a2f 100644
--- a/modules/libmar/tool/moz.build
+++ b/modules/libmar/tool/moz.build
@@ -20,12 +20,6 @@ if CONFIG["HOST_OS_ARCH"] == "WINNT":
         "ws2_32",
     ]
 
-# C11 for static_assert
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-HOST_CFLAGS += c11_flags
-
 HOST_DEFINES["NO_SIGN_VERIFY"] = True
 
 if CONFIG["MOZ_BUILD_APP"] != "tools/update-packaging":
@@ -33,8 +27,6 @@ if CONFIG["MOZ_BUILD_APP"] != "tools/update-packaging":
 
     SOURCES += HOST_SOURCES
 
-    CFLAGS += c11_flags
-
     USE_LIBS += [
         "mar",
         "nspr",
diff --git a/modules/libmar/verify/moz.build b/modules/libmar/verify/moz.build
index 5ea079a666bb..4b82e13e443e 100644
--- a/modules/libmar/verify/moz.build
+++ b/modules/libmar/verify/moz.build
@@ -44,9 +44,3 @@ LOCAL_INCLUDES += [
 if CONFIG["MOZ_USE_NSS_FOR_MAR"]:
     LOCAL_INCLUDES += ["../sign"]
     DEFINES["MAR_NSS"] = True
-
-# C11 for static_assert
-c11_flags = ["-std=gnu11"]
-if CONFIG["CC_TYPE"] == "clang-cl":
-    c11_flags.insert(0, "-Xclang")
-CFLAGS += c11_flags
diff --git a/toolkit/crashreporter/google-breakpad/src/common/mac/moz.build b/toolkit/crashreporter/google-breakpad/src/common/mac/moz.build
index 23fc2207a468..1ca3a642f1e2 100644
--- a/toolkit/crashreporter/google-breakpad/src/common/mac/moz.build
+++ b/toolkit/crashreporter/google-breakpad/src/common/mac/moz.build
@@ -22,6 +22,4 @@ SOURCES += [
 
 Library('breakpad_mac_common_s')
 
-CMFLAGS += ['-std=c99']
-
 include('/toolkit/crashreporter/crashreporter.mozbuild')