diff --git a/python/mozbuild/mozbuild/frontend/emitter.py b/python/mozbuild/mozbuild/frontend/emitter.py
index 940dfc2ab2a46..3acb662b4de11 100644
--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -132,6 +132,7 @@ def emit_from_sandbox(self, sandbox):
             JS_MODULES_PATH='JS_MODULES_PATH',
             LIBRARY_NAME='LIBRARY_NAME',
             LIBS='LIBS',
+            LIBXUL_LIBRARY='LIBXUL_LIBRARY',
             MODULE='MODULE',
             SDK_LIBRARY='SDK_LIBRARY',
             SHARED_LIBRARY_LIBS='SHARED_LIBRARY_LIBS',
diff --git a/python/mozbuild/mozbuild/frontend/sandbox_symbols.py b/python/mozbuild/mozbuild/frontend/sandbox_symbols.py
index 204cb7bb0f68f..e288b6068196e 100644
--- a/python/mozbuild/mozbuild/frontend/sandbox_symbols.py
+++ b/python/mozbuild/mozbuild/frontend/sandbox_symbols.py
@@ -209,6 +209,12 @@ def doc_to_paragraphs(doc):
         A list of libraries and flags to include when linking.
         """),
 
+    'LIBXUL_LIBRARY': (bool, bool, False,
+        """Whether the library in this directory is linked into libxul.
+
+        Implies MOZILLA_INTERNAL_API.
+        """),
+
     'SDK_LIBRARY': (StrictOrderingOnAppendList, list, [],
         """Elements of the distributed SDK.
 
diff --git a/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build b/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build
index 2ad897de7874a..f208953a9e44e 100644
--- a/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build
+++ b/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build
@@ -41,3 +41,4 @@ CMMSRCS = ['bar.mm', 'foo.mm']
 SSRCS = ['bar.S', 'foo.S']
 
 FAIL_ON_WARNINGS = True
+LIBXUL_LIBRARY = True
diff --git a/python/mozbuild/mozbuild/test/backend/test_recursivemake.py b/python/mozbuild/mozbuild/test/backend/test_recursivemake.py
index 9cd892b1d096c..23c3839d8e141 100644
--- a/python/mozbuild/mozbuild/test/backend/test_recursivemake.py
+++ b/python/mozbuild/mozbuild/test/backend/test_recursivemake.py
@@ -204,6 +204,9 @@ def test_variable_passthru(self):
             'LIBRARY_NAME': [
                 'LIBRARY_NAME := lib_name',
             ],
+            'LIBXUL_LIBRARY': [
+                'LIBXUL_LIBRARY := 1',
+            ],
             'SDK_LIBRARY': [
                 'SDK_LIBRARY += bar.sdk',
                 'SDK_LIBRARY += foo.sdk',
diff --git a/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build b/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build
index 8c34b775f4395..57db2a16aa13a 100644
--- a/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build
+++ b/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build
@@ -41,5 +41,6 @@ CMMSRCS = ['fans.mm', 'tans.mm']
 SSRCS = ['fans.S', 'tans.S']
 
 FAIL_ON_WARNINGS = True
+LIBXUL_LIBRARY = True
 
 NO_DIST_INSTALL = True
diff --git a/python/mozbuild/mozbuild/test/frontend/test_emitter.py b/python/mozbuild/mozbuild/test/frontend/test_emitter.py
index 37a72381a39cf..5737d4518fe52 100644
--- a/python/mozbuild/mozbuild/test/frontend/test_emitter.py
+++ b/python/mozbuild/mozbuild/test/frontend/test_emitter.py
@@ -145,6 +145,7 @@ def test_variable_passthru(self):
             HOST_LIBRARY_NAME='host_fans',
             LIBRARY_NAME='lib_name',
             LIBS=['fans.lib', 'tans.lib'],
+            LIBXUL_LIBRARY=True,
             NO_DIST_INSTALL='1',
             MODULE='module_name',
             SDK_LIBRARY=['fans.sdk', 'tans.sdk'],