--- a/browser/base/content/browser-sidebar.js
+++ b/browser/base/content/browser-sidebar.js
@@ -43,29 +43,36 @@ var SidebarUI = {
     return this.__title = document.getElementById("sidebar-title");
   },
   _splitter: null,
   _icon: null,
   _reversePositionButton: null,
   _switcherPanel: null,
   _switcherTarget: null,
   _switcherArrow: null,
+  _inited: false,
+
+  get initialized() {
+    return this._inited;
+  },
 
   init() {
     this._box = document.getElementById("sidebar-box");
     this._splitter = document.getElementById("sidebar-splitter");
     this._icon = document.getElementById("sidebar-icon");
     this._reversePositionButton = document.getElementById("sidebar-reverse-position");
     this._switcherPanel = document.getElementById("sidebarMenu-popup");
     this._switcherTarget = document.getElementById("sidebar-switcher-target");
     this._switcherArrow = document.getElementById("sidebar-switcher-arrow");
 
     this._switcherTarget.addEventListener("command", () => {
       this.toggleSwitcherPanel();
     });
+
+    this._inited = true;
   },
 
   uninit() {
     // If this is the last browser window, persist various values that should be
     // remembered for after a restart / reopening a browser window.
     let enumerator = Services.wm.getEnumerator("navigator:browser");
     if (!enumerator.hasMoreElements()) {
       document.persist("sidebar-box", "sidebarcommand");

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -5631,39 +5631,42 @@ var gUIDensity = {
     Services.prefs.setIntPref(this.uiDensityPref, mode);
   },
 
   update(mode) {
     if (mode == null) {
       mode = this.getCurrentDensity().mode;
     }
 
-    let docs = [
-      document.documentElement,
-      SidebarUI.browser.contentDocument.documentElement,
-    ];
+    let docs = [document.documentElement];
+    let shouldUpdateSidebar = SidebarUI.initialized && SidebarUI.isOpen;
+    if (shouldUpdateSidebar) {
+      docs.push(SidebarUI.browser.contentDocument.documentElement);
+    }
     for (let doc of docs) {
       switch (mode) {
       case this.MODE_COMPACT:
         doc.setAttribute("uidensity", "compact");
         break;
       case this.MODE_TOUCH:
         doc.setAttribute("uidensity", "touch");
         break;
       default:
         doc.removeAttribute("uidensity");
         break;
       }
     }
-    let tree = SidebarUI.browser.contentDocument.querySelector(".sidebar-placesTree");
-    if (tree) {
-      // Tree items don't update their styles without changing some property on the
-      // parent tree element, like background-color or border. See bug 1407399.
-      tree.style.border = "1px";
-      tree.style.border = "";
+    if (shouldUpdateSidebar) {
+      let tree = SidebarUI.browser.contentDocument.querySelector(".sidebar-placesTree");
+      if (tree) {
+        // Tree items don't update their styles without changing some property on the
+        // parent tree element, like background-color or border. See bug 1407399.
+        tree.style.border = "1px";
+        tree.style.border = "";
+      }
     }
 
     TabsInTitlebar.updateAppearance(true);
     gBrowser.tabContainer.uiDensityChanged();
   },
 };
 
 var gHomeButton = {