diff --git a/testing/marionette/client/marionette_driver/addons.py b/testing/marionette/client/marionette_driver/addons.py
index ebbe6fed4992..836be27ae4ae 100644
--- a/testing/marionette/client/marionette_driver/addons.py
+++ b/testing/marionette/client/marionette_driver/addons.py
@@ -31,28 +31,34 @@ class Addons:
     def __init__(self, marionette):
         self._mn = marionette
 
-    def install(self, path, temp=False):
-        """Install a Firefox addon.
-
-        If the addon is restartless, it can be used right away. Otherwise
-        a restart using :func:`~marionette_driver.marionette.Marionette.restart`
-        will be needed.
+    def install(self, path=None, temp=False, data=None):
+        """Install a Firefox addon, which can be used right away.
 
         :param path: A file path to the extension to be installed.
         :param temp: Install a temporary addon. Temporary addons will
                      automatically be uninstalled on shutdown and do not need
-                     to be signed, though they must be restartless.
+                     to be signed.
+        :param data: A base64-encoded string of a zip-packed addon.
 
         :returns: The addon ID string of the newly installed addon.
 
         :raises: :exc:`AddonInstallException`
 
         """
-        # On windows we can end up with a path with mixed \ and /
-        # which Firefox doesn't like
-        path = path.replace("/", os.path.sep)
 
-        body = {"path": path, "temporary": temp}
+        if (path and data) or (not path and not data):
+            raise AddonInstallException("Must use either path or data argument.")
+
+        body = {"temporary": temp}
+        if path:
+            # On windows we can end up with a path with mixed \ and /
+            # which Firefox doesn't like
+            path = path.replace("/", os.path.sep)
+            body.update({"path": path})
+
+        if data:
+            body.update({"addon": data})
+
         try:
             return self._mn._send_message("Addon:Install", body, key="value")
         except errors.UnknownException as e:
diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/executormarionette.py b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/executormarionette.py
index 08bc02cbc3c0..93b26a73c837 100644
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/executormarionette.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/executormarionette.py
@@ -47,6 +47,7 @@ from .protocol import (AccessibilityProtocolPart,
                        DevicePostureProtocolPart,
                        VirtualPressureSourceProtocolPart,
                        DisplayFeaturesProtocolPart,
+                       WebExtensionsProtocolPart,
                        merge_dicts)
 
 
@@ -738,6 +739,24 @@ class MarionetteDisplayFeaturesProtocolPart(DisplayFeaturesProtocolPart):
     def clear_display_features(self):
         raise NotImplementedError("clear_display_features not yet implemented")
 
+
+class MarionetteWebExtensionsProtocolPart(WebExtensionsProtocolPart):
+    def setup(self):
+        self.addons = Addons(self.parent.marionette)
+
+    def install_web_extension(self, extension):
+        if extension["type"] == "base64":
+            extension_id = self.addons.install(data=extension["value"], temp=True)
+        else:
+            path = self.parent.test_dir + extension["path"]
+            extension_id = self.addons.install(path, temp=True)
+
+        return {'extension': extension_id}
+
+    def uninstall_web_extension(self, extension_id):
+        return self.addons.uninstall(extension_id)
+
+
 class MarionetteProtocol(Protocol):
     implements = [MarionetteBaseProtocolPart,
                   MarionetteTestharnessProtocolPart,
@@ -761,7 +780,8 @@ class MarionetteProtocol(Protocol):
                   MarionetteVirtualSensorProtocolPart,
                   MarionetteDevicePostureProtocolPart,
                   MarionetteVirtualPressureSourceProtocolPart,
-                  MarionetteDisplayFeaturesProtocolPart]
+                  MarionetteDisplayFeaturesProtocolPart,
+                  MarionetteWebExtensionsProtocolPart]
 
     def __init__(self, executor, browser, capabilities=None, timeout_multiplier=1, e10s=True, ccov=False):
         do_delayed_imports()
@@ -955,6 +975,10 @@ class MarionetteTestharnessExecutor(TestharnessExecutor):
             self.protocol.testharness.load_runner(new_environment["protocol"])
 
     def do_test(self, test):
+        # TODO: followup to do this properly, pass test as state on the CallbackHandler:
+        # https://phabricator.services.mozilla.com/D244400?id=1030480#inline-1369921
+        self.protocol.test_dir = os.path.dirname(test.path)
+
         timeout = (test.timeout * self.timeout_multiplier if self.debug_info is None
                    else None)
 
diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/protocol.py b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/protocol.py
index 6b9a0de9bb24..af32f487c953 100644
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/protocol.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/protocol.py
@@ -335,6 +335,21 @@ class AccessibilityProtocolPart(ProtocolPart):
         pass
 
 
+class WebExtensionsProtocolPart(ProtocolPart):
+    """Protocol part for managing WebExtensions"""
+    __metaclass__ = ABCMeta
+
+    name = "web_extensions"
+
+    @abstractmethod
+    def install_web_extension(self, extension):
+        pass
+
+    @abstractmethod
+    def uninstall_web_extension(self, extension_id):
+        pass
+
+
 class BidiBluetoothProtocolPart(ProtocolPart):
     """Protocol part for managing BiDi events"""
     __metaclass__ = ABCMeta