--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -227,16 +227,20 @@ pref("general.skins.selectedSkin", "clas
 
 pref("general.smoothScroll", true);
 #ifdef UNIX_BUT_NOT_MAC
 pref("general.autoScroll", false);
 #else
 pref("general.autoScroll", true);
 #endif
 
+// UI density of the browser chrome. This mostly affects toolbarbutton
+// and urlbar spacing. The possible values are 0=normal, 1=compact, 2=touch.
+pref("browser.uidensity", 0);
+
 // At startup, check if we're the default browser and prompt user if not.
 pref("browser.shell.checkDefaultBrowser", true);
 pref("browser.shell.shortcutFavicons",true);
 pref("browser.shell.mostRecentDateSetAsDefault", "");
 pref("browser.shell.skipDefaultBrowserCheckOnFirstRun", true);
 pref("browser.shell.didSkipDefaultBrowserCheckOnFirstRun", false);
 pref("browser.shell.defaultBrowserCheckCount", 0);
 pref("browser.defaultbrowser.notificationbar", false);

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1309,16 +1309,19 @@ var gBrowserInit = {
     SidebarUI.init();
 
     // Certain kinds of automigration rely on this notification to complete
     // their tasks BEFORE the browser window is shown. SessionStore uses it to
     // restore tabs into windows AFTER important parts like gMultiProcessBrowser
     // have been initialized.
     Services.obs.notifyObservers(window, "browser-window-before-show");
 
+    gUIDensity.update();
+    gPrefService.addObserver(gUIDensity.prefDomain, gUIDensity);
+
     let isResistFingerprintingEnabled = gPrefService.getBoolPref("privacy.resistFingerprinting");
 
     // Set a sane starting width/height for all resolutions on new profiles.
     if (isResistFingerprintingEnabled) {
       // When the fingerprinting resistance is enabled, making sure that we don't
       // have a maximum window to interfere with generating rounded window dimensions.
       document.documentElement.setAttribute("sizemode", "normal");
     } else if (!document.documentElement.hasAttribute("width")) {
@@ -1761,16 +1764,18 @@ var gBrowserInit = {
     FullScreen.uninit();
 
     gSync.uninit();
 
     gExtensionsNotifications.uninit();
 
     Services.obs.removeObserver(gPluginHandler.NPAPIPluginCrashed, "plugin-crashed");
 
+    gPrefService.removeObserver(gUIDensity.prefDomain, gUIDensity);
+
     try {
       gBrowser.removeProgressListener(window.XULBrowserWindow);
       gBrowser.removeTabsProgressListener(window.TabsProgressListener);
     } catch (ex) {
     }
 
     PlacesToolbarHelper.uninit();
 
@@ -5405,16 +5410,41 @@ var gTabletModePageCounter = {
     }
   },
 };
 
 function displaySecurityInfo() {
   BrowserPageInfo(null, "securityTab");
 }
 
+// Updates the UI density (for touch and compact mode) based on the uidensity pref.
+var gUIDensity = {
+  prefDomain: "browser.uidensity",
+  observe(aSubject, aTopic, aPrefName) {
+    if (aTopic != "nsPref:changed" || aPrefName != this.prefDomain)
+      return;
+
+    this.update();
+  },
+
+  update() {
+    let doc = document.documentElement;
+    switch (gPrefService.getIntPref(this.prefDomain)) {
+    case 1:
+      doc.setAttribute("uidensity", "compact");
+      break;
+    case 2:
+      doc.setAttribute("uidensity", "touch");
+      break;
+    default:
+      doc.removeAttribute("uidensity");
+      break;
+    }
+  },
+};
 
 var gHomeButton = {
   prefDomain: "browser.startup.homepage",
   observe(aSubject, aTopic, aPrefName) {
     if (aTopic != "nsPref:changed" || aPrefName != this.prefDomain)
       return;
 
     this.updateTooltip();