diff --git a/svc/src/hudctrl/hud.py b/svc/src/hudctrl/hud.py
index d76891c..4e8618c 100644
--- a/svc/src/hudctrl/hud.py
+++ b/svc/src/hudctrl/hud.py
@@ -35,19 +35,21 @@ class HUDService:
         self.windows: Dict[str, str] = {}
 
         self.urls_file = Path('urls.json')
+        self.lock = asyncio.Lock()
 
     async def get_screens(self) -> Dict[str, HUDScreen]:
         assert self.marionette
         screens = {}
-        for name, handle in self.windows.items():
-            await self.marionette.switch_to_window(handle)
-            title = await self.marionette.get_title()
-            url = await self.marionette.get_url()
-            rect = await self.marionette.get_window_rect()
-            screens[name] = HUDScreen(
-                handle=handle, title=title, url=url, dimensions=rect
-            )
-        return screens
+        async with self.lock:
+            for name, handle in self.windows.items():
+                await self.marionette.switch_to_window(handle)
+                title = await self.marionette.get_title()
+                url = await self.marionette.get_url()
+                rect = await self.marionette.get_window_rect()
+                screens[name] = HUDScreen(
+                    handle=handle, title=title, url=url, dimensions=rect
+                )
+            return screens
 
     async def initialize(self) -> None:
         assert self.marionette
@@ -56,36 +58,39 @@ class HUDService:
             raise NoMonitorConfig(
                 'Cannot initialize display: No monitor config supplied'
             )
-        log.info(
-            'Initializing display for %d monitors',
-            len(self.monitor_config.monitors),
-        )
-        window: Optional[str] = await self.marionette.new_window('window')
-        for handle in await self.marionette.get_window_handles():
-            if handle == window:
-                continue
-            await self.marionette.switch_to_window(handle)
-            await self.marionette.close_window()
-        tasks = []
-        for monitor in self.monitor_config.monitors:
-            try:
-                url = self.urls[monitor.name]
-            except KeyError:
-                continue
-            if window is None:
-                window = await self.marionette.new_window('window')
-            self.windows[monitor.name] = window
-            await self.marionette.switch_to_window(window)
-            await self.marionette.set_window_rect(
-                x=monitor.pos_x,
-                y=1,
+        async with self.lock:
+            log.info(
+                'Initializing display for %d monitors',
+                len(self.monitor_config.monitors),
             )
-            await self.marionette.fullscreen()
-            log.info('Screen %s: Opening URL %s', monitor.name, url)
-            tasks.append(asyncio.create_task(self.marionette.navigate(url)))
-            window = None
-        if tasks:
-            await asyncio.wait(tasks)
+            window: Optional[str] = await self.marionette.new_window('window')
+            for handle in await self.marionette.get_window_handles():
+                if handle == window:
+                    continue
+                await self.marionette.switch_to_window(handle)
+                await self.marionette.close_window()
+            tasks = []
+            for monitor in self.monitor_config.monitors:
+                try:
+                    url = self.urls[monitor.name]
+                except KeyError:
+                    continue
+                if window is None:
+                    window = await self.marionette.new_window('window')
+                self.windows[monitor.name] = window
+                await self.marionette.switch_to_window(window)
+                await self.marionette.set_window_rect(
+                    x=monitor.pos_x,
+                    y=1,
+                )
+                await self.marionette.fullscreen()
+                log.info('Screen %s: Opening URL %s', monitor.name, url)
+                tasks.append(
+                    asyncio.create_task(self.marionette.navigate(url))
+                )
+                window = None
+            if tasks:
+                await asyncio.wait(tasks)
 
     def load_urls(self) -> None:
         try:
@@ -113,8 +118,9 @@ class HUDService:
 
     async def refresh_screen(self, name: str) -> None:
         assert self.marionette
-        await self.marionette.switch_to_window(self.windows[name])
-        await self.marionette.refresh()
+        async with self.lock:
+            await self.marionette.switch_to_window(self.windows[name])
+            await self.marionette.refresh()
 
     async def set_display(self, host: str, port: int) -> None:
         if self.marionette:
@@ -138,6 +144,7 @@ class HUDService:
     async def take_screenshot(self, screen: str) -> bytes:
         assert self.marionette
         handle = self.windows[screen]
-        await self.marionette.switch_to_window(handle)
-        data = await self.marionette.take_screenshot()
+        async with self.lock:
+            await self.marionette.switch_to_window(handle)
+            data = await self.marionette.take_screenshot()
         return base64.b64decode(data)