diff --git a/main.js b/main.js
index 44b274f..4124ba9 100644
--- a/main.js
+++ b/main.js
@@ -151,6 +151,77 @@ const GPUConfig = require('./gpu-config');
 const PluginManager = require('./plugin-manager');
 const portableData = require('./portable-data');
 
+// Windows: set explicit AppUserModelID to ensure proper default-app registration
+// and notification branding.
+if (process.platform === 'win32') {
+  try {
+    app.setAppUserModelId('com.andrewzambazos.nebula');
+  } catch {}
+}
+
+// --- Single instance + protocol URL handling ---
+let pendingOpenUrl = null;
+
+function extractUrlFromArgv(argv = []) {
+  return argv.find(arg => /^https?:\/\//i.test(arg));
+}
+
+function openUrlInExistingWindow(targetUrl) {
+  if (!targetUrl) return false;
+  const windows = BrowserWindow.getAllWindows();
+  const mainWindow = windows.find(w => {
+    try { return w && !w.isDestroyed() && !w.getParentWindow(); } catch { return false; }
+  });
+
+  if (mainWindow) {
+    try { mainWindow.show(); } catch {}
+    try { mainWindow.focus(); } catch {}
+    try {
+      mainWindow.webContents.send('open-url-new-tab', targetUrl);
+      return true;
+    } catch {}
+    try {
+      mainWindow.webContents.send('open-url', targetUrl);
+      return true;
+    } catch {}
+  }
+
+  pendingOpenUrl = targetUrl;
+  return false;
+}
+
+const gotSingleInstanceLock = app.requestSingleInstanceLock();
+if (!gotSingleInstanceLock) {
+  app.quit();
+} else {
+  app.on('second-instance', (_event, argv) => {
+    const url = extractUrlFromArgv(argv);
+    if (url) {
+      openUrlInExistingWindow(url);
+      return;
+    }
+    const windows = BrowserWindow.getAllWindows();
+    const mainWindow = windows.find(w => {
+      try { return w && !w.isDestroyed() && !w.getParentWindow(); } catch { return false; }
+    });
+    if (mainWindow) {
+      try { mainWindow.show(); } catch {}
+      try { mainWindow.focus(); } catch {}
+    }
+  });
+}
+
+app.on('open-url', (event, url) => {
+  event.preventDefault();
+  openUrlInExistingWindow(url);
+});
+
+// Capture protocol URL if the app was launched with one
+const initialProtocolUrl = extractUrlFromArgv(process.argv);
+if (initialProtocolUrl) {
+  pendingOpenUrl = initialProtocolUrl;
+}
+
 // Initialize performance monitoring and GPU management
 const perfMonitor = new PerformanceMonitor();
 const gpuFallback = new GPUFallback();
@@ -555,8 +626,21 @@ async function completeFirstRun(preferences = {}) {
 /**
  * Check if Nebula is set as the default browser
  */
+function getProtocolClientArgs() {
+  if (process.platform === 'win32' && process.defaultApp) {
+    const appPath = path.resolve(process.argv[1]);
+    return { exe: process.execPath, args: [appPath] };
+  }
+  return null;
+}
+
 function isDefaultBrowser() {
   try {
+    const protocolArgs = getProtocolClientArgs();
+    if (protocolArgs) {
+      return app.isDefaultProtocolClient('http', protocolArgs.exe, protocolArgs.args)
+        && app.isDefaultProtocolClient('https', protocolArgs.exe, protocolArgs.args);
+    }
     return app.isDefaultProtocolClient('http') && app.isDefaultProtocolClient('https');
   } catch (err) {
     console.error('[DefaultBrowser] Error checking default browser status:', err);
@@ -569,18 +653,42 @@ function isDefaultBrowser() {
  */
 function setAsDefaultBrowser() {
   try {
-    const httpResult = app.setAsDefaultProtocolClient('http');
-    const httpsResult = app.setAsDefaultProtocolClient('https');
-    const htmlResult = app.setAsDefaultProtocolClient('html');
-    
-    console.log('[DefaultBrowser] Set as default:', { httpResult, httpsResult, htmlResult });
-    return httpResult && httpsResult;
+    const protocolArgs = getProtocolClientArgs();
+    const httpResult = protocolArgs
+      ? app.setAsDefaultProtocolClient('http', protocolArgs.exe, protocolArgs.args)
+      : app.setAsDefaultProtocolClient('http');
+    const httpsResult = protocolArgs
+      ? app.setAsDefaultProtocolClient('https', protocolArgs.exe, protocolArgs.args)
+      : app.setAsDefaultProtocolClient('https');
+    const htmlResult = protocolArgs
+      ? app.setAsDefaultProtocolClient('html', protocolArgs.exe, protocolArgs.args)
+      : app.setAsDefaultProtocolClient('html');
+
+    const success = httpResult && httpsResult;
+    const needsUserAction = success && !isDefaultBrowser();
+
+    console.log('[DefaultBrowser] Set as default:', { httpResult, httpsResult, htmlResult, needsUserAction });
+    return { success, needsUserAction };
   } catch (err) {
     console.error('[DefaultBrowser] Error setting as default browser:', err);
-    return false;
+    return { success: false, needsUserAction: false, error: err.message };
   }
 }
 
+function openDefaultBrowserSettings() {
+  try {
+    if (process.platform === 'win32') {
+      return shell.openExternal('ms-settings:defaultapps');
+    }
+    if (process.platform === 'darwin') {
+      return shell.openExternal('x-apple.systempreferences:com.apple.preference.general?DefaultWebBrowser');
+    }
+  } catch (err) {
+    console.warn('[DefaultBrowser] Failed to open system settings:', err.message || err);
+  }
+  return false;
+}
+
 // =============================================================================
 
 // Initialize portable data paths BEFORE app.ready (must be done early)
@@ -1246,12 +1354,15 @@ app.whenReady().then(() => {
 
   // If launched via SteamOS Gaming Mode / gamepad UI, default to Big Picture Mode.
   // Desktop launches remain unchanged. Big Picture now opens in main window to keep resources low.
-  const startInBigPicture = shouldStartInBigPictureMode();
+  const startUrl = pendingOpenUrl;
+  pendingOpenUrl = null;
+
+  const startInBigPicture = startUrl ? false : shouldStartInBigPictureMode();
   if (startInBigPicture) {
     console.log('[Startup] Detected game mode launch; starting in Big Picture Mode (in main window)');
     createWindow(null, true); // Pass bigPictureMode flag
   } else {
-    createWindow();
+    createWindow(startUrl || null, false);
   }
 
   // Initialize user plugins after app ready
@@ -1464,13 +1575,23 @@ ipcMain.handle('is-default-browser', () => {
 ipcMain.handle('set-as-default-browser', () => {
   try {
     const result = setAsDefaultBrowser();
-    return { success: result };
+    return result;
   } catch (err) {
     console.error('[DefaultBrowser] Error in IPC handler:', err);
     return { success: false, error: err.message };
   }
 });
 
+ipcMain.handle('open-default-browser-settings', () => {
+  try {
+    const result = openDefaultBrowserSettings();
+    return { success: !!result };
+  } catch (err) {
+    console.error('[DefaultBrowser] Error opening system settings:', err);
+    return { success: false, error: err.message };
+  }
+});
+
 // --- window control handlers (only registered once now)
 ipcMain.handle('window-minimize', event => {
   BrowserWindow.fromWebContents(event.sender).minimize();
diff --git a/package.json b/package.json
index c5cdedb..06381bf 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,15 @@
   },
   "build": {
     "appId": "com.andrewzambazos.nebula",
+    "protocols": [
+      {
+        "name": "Nebula",
+        "schemes": [
+          "http",
+          "https"
+        ]
+      }
+    ],
     "publish": [
       {
         "provider": "github",
diff --git a/preload.js b/preload.js
index 0cefaba..8eb4684 100644
--- a/preload.js
+++ b/preload.js
@@ -517,6 +517,8 @@ contextBridge.exposeInMainWorld('api', {
   isDefaultBrowser: () => ipcRenderer.invoke('is-default-browser'),
   // Set Nebula as the default browser
   setAsDefaultBrowser: () => ipcRenderer.invoke('set-as-default-browser'),
+  // Open OS default browser settings
+  openDefaultBrowserSettings: () => ipcRenderer.invoke('open-default-browser-settings'),
   // Complete first-run setup
   completeFirstRun: (data) => ipcRenderer.invoke('complete-first-run', data),
   // Get first-run data
diff --git a/renderer/setup.js b/renderer/setup.js
index a3d5e85..ba22562 100644
--- a/renderer/setup.js
+++ b/renderer/setup.js
@@ -294,26 +294,47 @@ async function setDefaultBrowser() {
     const result = await window.api.setAsDefaultBrowser();
     
     if (result.success) {
-      setupState.defaultBrowserSet = true;
-      
+      const isDefault = await window.api.isDefaultBrowser();
+      if (isDefault) {
+        setupState.defaultBrowserSet = true;
+        
+        if (statusEl) {
+          statusEl.classList.remove('not-default');
+          statusEl.classList.add('is-default');
+          statusEl.innerHTML = `
+            <div class="status-icon">✓</div>
+            <p class="status-text">Nebula is now your default browser!</p>
+          `;
+        }
+        
+        if (btn) {
+          btn.innerHTML = '<span class="btn-icon">✓</span> Set Successfully';
+        }
+        
+        // Auto-advance after a brief delay
+        setTimeout(() => goToStep(4), 1500);
+        return;
+      }
+
       if (statusEl) {
         statusEl.classList.remove('not-default');
-        statusEl.classList.add('is-default');
         statusEl.innerHTML = `
-          <div class="status-icon">✓</div>
-          <p class="status-text">Nebula is now your default browser!</p>
+          <div class="status-icon">ℹ️</div>
+          <p class="status-text">System settings opened. Choose Nebula as your default browser to finish.</p>
         `;
       }
-      
+
       if (btn) {
-        btn.innerHTML = '<span class="btn-icon">✓</span> Set Successfully';
+        btn.disabled = false;
+        btn.innerHTML = '<span class="btn-icon">↻</span> Check Again';
       }
-      
-      // Auto-advance after a brief delay
-      setTimeout(() => goToStep(4), 1500);
-    } else {
-      throw new Error(result.error || 'Failed to set default browser');
+
+      if (result.needsUserAction && window.api.openDefaultBrowserSettings) {
+        try { await window.api.openDefaultBrowserSettings(); } catch {}
+      }
+      return;
     }
+    throw new Error(result.error || 'Failed to set default browser');
   } catch (error) {
     console.error('[Setup] Error setting default browser:', error);