--- a/browser/base/content/browser-fullZoom.js
+++ b/browser/base/content/browser-fullZoom.js
@@ -32,16 +32,21 @@ var FullZoom = {
     if (this._siteSpecificPref === undefined) {
       this._siteSpecificPref =
         !gPrefService.getBoolPref("privacy.resistFingerprinting") &&
         gPrefService.getBoolPref("browser.zoom.siteSpecific");
     }
     return this._siteSpecificPref;
   },
 
+  // @see ZoomUI.updateZoomUI and ext-tabs._getZoomSettings
+  get defaultUserZoom() {
+    return Number(this._ensureValid(this._globalValue).toFixed(2));
+  },
+
   // nsISupports
 
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIDOMEventListener,
                                          Ci.nsIObserver,
                                          Ci.nsIContentPrefObserver,
                                          Ci.nsISupportsWeakReference,
                                          Ci.nsISupports]),
 
@@ -50,21 +55,29 @@ var FullZoom = {
   init: function FullZoom_init() {
     gBrowser.addEventListener("ZoomChangeUsingMouseWheel", this);
 
     // Register ourselves with the service so we know when our pref changes.
     this._cps2 = Cc["@mozilla.org/content-pref/service;1"].
                  getService(Ci.nsIContentPrefService2);
     this._cps2.addObserverForName(this.name, this);
 
+    if (!("_globalValue" in this)){ // @see this._getGlobalValue
+      // Get user default zoom from pref
+      let value=gPrefService.getCharPref("browser.zoom.userDefaultFullZoom","");
+      // Set default zoom only if we have the pref
+      if (value) this._setDefaultUserZoom(value);
+    }
+
     this.updateBackgroundTabs =
       gPrefService.getBoolPref("browser.zoom.updateBackgroundTabs");
 
     // Listen for changes to the browser.zoom branch so we can enable/disable
-    // updating background tabs and per-site saving and restoring of zoom levels.
+    // updating background tabs and per-site saving and restoring of zoom levels,
+    // and set user default zoom levels.
     gPrefService.addObserver("browser.zoom.", this, true);
 
     // Also need to listen to privacy.resistFingerprinting in order to update
     // this._siteSpecificPref.
     gPrefService.addObserver("privacy.resistFingerprinting", this, true);
 
     // If we received onLocationChange events for any of the current browsers
     // before we were initialized we want to replay those upon initialization.
@@ -79,16 +92,29 @@ var FullZoom = {
   },
 
   destroy: function FullZoom_destroy() {
     gPrefService.removeObserver("browser.zoom.", this);
     this._cps2.removeObserverForName(this.name, this);
     gBrowser.removeEventListener("ZoomChangeUsingMouseWheel", this);
   },
 
+  _setDefaultUserZoom: function FullZoom__setDefaultUserZoom(aValue){
+    // "Snap" to one of the toolkit.zoomManager.zoomValues
+    let value = ZoomManager.snap(this._ensureValid(parseFloat(aValue)));
+    let browser = gBrowser.selectedBrowser;
+    // this._onContentPrefChanged should take care of the rest, since
+    // we are the nsIContentPrefObserver
+    if (value!=1) {
+      this._cps2.setGlobal(this.name,value,this._loadContextFromBrowser(browser));
+    }else{
+      // 1 is the default value... might as well remove the nsIContentPrefService2 global
+      this._cps2.removeGlobal(this.name,this._loadContextFromBrowser(browser));
+    }
+  },
 
   // Event Handlers
 
   // nsIDOMEventListener
 
   handleEvent: function FullZoom_handleEvent(event) {
     switch (event.type) {
       case "ZoomChangeUsingMouseWheel":
@@ -110,16 +136,20 @@ var FullZoom = {
           case "browser.zoom.siteSpecific":
             // Invalidate pref cache.
             this._siteSpecificPref = undefined;
             break;
           case "browser.zoom.updateBackgroundTabs":
             this.updateBackgroundTabs =
               gPrefService.getBoolPref("browser.zoom.updateBackgroundTabs");
             break;
+          case "browser.zoom.userDefaultFullZoom":
+            let value=gPrefService.getCharPref("browser.zoom.userDefaultFullZoom");
+            this._setDefaultUserZoom(value);
+            break;
         }
         break;
     }
   },
 
   // nsIContentPrefObserver
 
   onContentPrefSet: function FullZoom_onContentPrefSet(aGroup, aName, aValue, aIsPrivate) {
@@ -368,23 +398,29 @@ var FullZoom = {
    * @param browser  The zoom of this browser will be saved.  Required.
    */
   _applyZoomToPref: function FullZoom__applyZoomToPref(browser) {
     if (!this.siteSpecific ||
         gInPrintPreviewMode ||
         browser.isSyntheticDocument)
       return;
 
-    this._cps2.set(browser.currentURI.spec, this.name,
-                   ZoomManager.getZoomForBrowser(browser),
-                   this._loadContextFromBrowser(browser), {
-      handleCompletion: () => {
-        this._isNextContentPrefChangeInternal = true;
-      },
-    });
+    // Is it better(cleaner) to remove the pref if we are at the default zoom ???
+    let zoom=ZoomManager.getZoomForBrowser(browser);
+    if (zoom!==this._ensureValid(this._globalValue)) {
+      this._cps2.set(browser.currentURI.spec, this.name,
+                     zoom,
+                     this._loadContextFromBrowser(browser), {
+        handleCompletion: () => {
+          this._isNextContentPrefChangeInternal = true;
+        },
+      });
+    }else
+      this._removePref(browser);
+
   },
 
   /**
    * Removes from the content prefs store the zoom level of the given browser.
    *
    * @param browser  The zoom of this browser will be removed.  Required.
    */
   _removePref: function FullZoom__removePref(browser) {

--- a/browser/components/extensions/ext-tabs.js
+++ b/browser/components/extensions/ext-tabs.js
@@ -714,17 +714,17 @@ this.tabs = class extends ExtensionAPI {
         _getZoomSettings(tabId) {
           let nativeTab = getTabOrActive(tabId);
 
           let {FullZoom} = nativeTab.ownerGlobal;
 
           return {
             mode: "automatic",
             scope: FullZoom.siteSpecific ? "per-origin" : "per-tab",
-            defaultZoomFactor: 1,
+            defaultZoomFactor: 1, // <-- Should this be FullZoom.defaultUserZoom ???
           };
         },
 
         getZoomSettings(tabId) {
           return Promise.resolve(this._getZoomSettings(tabId));
         },
 
         setZoomSettings(tabId, settings) {

--- a/browser/modules/ZoomUI.jsm
+++ b/browser/modules/ZoomUI.jsm
@@ -71,20 +71,20 @@ function updateZoomUI(aBrowser, aAnimate
   }
 
   let appMenuZoomReset = win.document.getElementById("appMenu-zoomReset-button");
   let customizableZoomControls = win.document.getElementById("zoom-controls");
   let customizableZoomReset = win.document.getElementById("zoom-reset-button");
   let urlbarZoomButton = win.document.getElementById("urlbar-zoom-button");
   let zoomFactor = Math.round(win.ZoomManager.zoom * 100);
 
-  // Hide urlbar zoom button if zoom is at 100% or the customizable control is
+  // Hide urlbar zoom button if zoom is at (User)Default Value or the customizable control is
   // in the toolbar.
   urlbarZoomButton.hidden =
-    (zoomFactor == 100 ||
+    (zoomFactor == Math.round(win.FullZoom.defaultUserZoom*100) ||
      (customizableZoomControls &&
       customizableZoomControls.getAttribute("cui-areatype") == "toolbar"));
 
   let label = win.gNavigatorBundle.getFormattedString("zoom-button.label", [zoomFactor]);
   if (appMenuZoomReset) {
     appMenuZoomReset.setAttribute("label", label);
   }
   if (customizableZoomReset) {