--- a/browser/base/content/tabbrowser.js
+++ b/browser/base/content/tabbrowser.js
@@ -151,19 +151,16 @@ window._gBrowser = {
 
     this._soundPlayingAttrRemovalTimer = 0;
 
     this._hoverTabTimer = null;
 
     this.mCurrentBrowser = this.initialBrowser;
     this.mCurrentBrowser.permanentKey = {};
 
-    CustomizableUI.addListener(this);
-    this._updateNewTabVisibility();
-
     Services.obs.addObserver(this, "contextual-identity-updated");
 
     this.mCurrentTab = this.tabContainer.firstChild;
     const nsIEventListenerService =
       Ci.nsIEventListenerService;
     let els = Cc["@mozilla.org/eventlistenerservice;1"]
       .getService(nsIEventListenerService);
     els.addSystemEventListener(document, "keydown", this, false);
@@ -3894,52 +3891,16 @@ window._gBrowser = {
       {
         // This is the only pref observed.
         this._findAsYouType = Services.prefs.getBoolPref("accessibility.typeaheadfind");
         break;
       }
     }
   },
 
-  _updateNewTabVisibility() {
-    // Helper functions to help deal with customize mode wrapping some items
-    let wrap = n => n.parentNode.localName == "toolbarpaletteitem" ? n.parentNode : n;
-    let unwrap = n => n && n.localName == "toolbarpaletteitem" ? n.firstElementChild : n;
-
-    let sib = this.tabContainer;
-    do {
-      sib = unwrap(wrap(sib).nextElementSibling);
-    } while (sib && sib.hidden);
-
-    const kAttr = "hasadjacentnewtabbutton";
-    if (sib && sib.id == "new-tab-button") {
-      this.tabContainer.setAttribute(kAttr, "true");
-    } else {
-      this.tabContainer.removeAttribute(kAttr);
-    }
-  },
-
-  onWidgetAfterDOMChange(aNode, aNextNode, aContainer) {
-    if (aContainer.ownerDocument == document &&
-        aContainer.id == "TabsToolbar") {
-      this._updateNewTabVisibility();
-    }
-  },
-
-  onAreaNodeRegistered(aArea, aContainer) {
-    if (aContainer.ownerDocument == document &&
-        aArea == "TabsToolbar") {
-      this._updateNewTabVisibility();
-    }
-  },
-
-  onAreaReset(aArea, aContainer) {
-    this.onAreaNodeRegistered(aArea, aContainer);
-  },
-
   _generateUniquePanelID() {
     if (!this._uniquePanelIDCounter) {
       this._uniquePanelIDCounter = 0;
     }
 
     let outerID = window.QueryInterface(Ci.nsIInterfaceRequestor)
                         .getInterface(Ci.nsIDOMWindowUtils)
                         .outerWindowID;
@@ -3948,18 +3909,16 @@ window._gBrowser = {
     // window ID. We switched to a monotonic counter as Date.now() lead
     // to random failures because of colliding IDs.
     return "panel-" + outerID + "-" + (++this._uniquePanelIDCounter);
   },
 
   destroy() {
     Services.obs.removeObserver(this, "contextual-identity-updated");
 
-    CustomizableUI.removeListener(this);
-
     for (let tab of this.tabs) {
       let browser = tab.linkedBrowser;
       if (browser.registeredOpenURI) {
         this._unifiedComplete.unregisterOpenPage(browser.registeredOpenURI,
           browser.getAttribute("usercontextid") || 0);
         delete browser.registeredOpenURI;
       }
 

--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -121,22 +121,27 @@
 
           window.addEventListener("resize", this);
           window.addEventListener("DOMContentLoaded", this);
 
           Services.prefs.addObserver("privacy.userContext", this);
           this.observe(null, "nsPref:changed", "privacy.userContext.enabled");
 
           this._setPositionalAttributes();
+
+          CustomizableUI.addListener(this);
+          this._updateNewTabVisibility();
         ]]>
       </constructor>
 
       <destructor>
         <![CDATA[
           Services.prefs.removeObserver("privacy.userContext", this);
+
+          CustomizableUI.removeListener(this);
         ]]>
       </destructor>
 
       <field name="tabbox" readonly="true">
         this.tabbrowser.tabbox;
       </field>
 
       <field name="contextMenu" readonly="true">
@@ -903,16 +908,67 @@
           // If the tab's browser is lazy, we need to `_insertBrowser` in order
           // to have a linkedPanel.  This will also serve to bind the browser
           // and make it ready to use when the tab is selected.
           this.tabbrowser._insertBrowser(aTab);
           return document.getElementById(aTab.linkedPanel);
         ]]>
         </body>
       </method>
+
+      <method name="_updateNewTabVisibility">
+        <body><![CDATA[
+          // Helper functions to help deal with customize mode wrapping some items
+          let wrap = n => n.parentNode.localName == "toolbarpaletteitem" ? n.parentNode : n;
+          let unwrap = n => n && n.localName == "toolbarpaletteitem" ? n.firstElementChild : n;
+
+          let sib = this;
+          do {
+            sib = unwrap(wrap(sib).nextElementSibling);
+          } while (sib && sib.hidden);
+
+          const kAttr = "hasadjacentnewtabbutton";
+          if (sib && sib.id == "new-tab-button") {
+            this.setAttribute(kAttr, "true");
+          } else {
+            this.removeAttribute(kAttr);
+          }
+        ]]></body>
+      </method>
+
+      <method name="onWidgetAfterDOMChange">
+        <parameter name="aNode"/>
+        <parameter name="aNextNode"/>
+        <parameter name="aContainer"/>
+        <body><![CDATA[
+          if (aContainer.ownerDocument == document &&
+              aContainer.id == "TabsToolbar") {
+            this._updateNewTabVisibility();
+          }
+        ]]></body>
+      </method>
+
+      <method name="onAreaNodeRegistered">
+        <parameter name="aArea"/>
+        <parameter name="aContainer"/>
+        <body><![CDATA[
+          if (aContainer.ownerDocument == document &&
+              aArea == "TabsToolbar") {
+            this._updateNewTabVisibility();
+          }
+        ]]></body>
+      </method>
+
+      <method name="onAreaReset">
+        <parameter name="aArea"/>
+        <parameter name="aContainer"/>
+        <body><![CDATA[
+          this.onAreaNodeRegistered(aArea, aContainer);
+        ]]></body>
+      </method>
     </implementation>
 
     <handlers>
       <handler event="TabSelect" action="this._handleTabSelect();"/>
 
       <handler event="transitionend"><![CDATA[
         if (event.propertyName != "max-width")
           return;