--- a/browser/base/content/tabbrowser.js
+++ b/browser/base/content/tabbrowser.js
@@ -85,17 +85,17 @@ window._gBrowser = {
   mProgressListeners: [],
 
   mTabsProgressListeners: [],
 
   _tabListeners: new Map(),
 
   _tabFilters: new Map(),
 
-  mIsBusy: false,
+  _isBusy: false,
 
   _outerWindowIDBrowserMap: new Map(),
 
   arrowKeysShouldWrap: AppConstants == "macosx",
 
   _autoScrollPopup: null,
 
   _previewMode: false,
@@ -1021,29 +1021,29 @@ window._gBrowser = {
       //     to dispatch an event.
       newTab.finishUnselectedTabHoverTimer();
       newBrowser.unselectedTabHover(false);
     }
 
     // If the new tab is busy, and our current state is not busy, then
     // we need to fire a start to all progress listeners.
     const nsIWebProgressListener = Ci.nsIWebProgressListener;
-    if (newTab.hasAttribute("busy") && !this.mIsBusy) {
-      this.mIsBusy = true;
+    if (newTab.hasAttribute("busy") && !this._isBusy) {
+      this._isBusy = true;
       this._callProgressListeners(null, "onStateChange",
                                   [webProgress, null,
                                   nsIWebProgressListener.STATE_START |
                                   nsIWebProgressListener.STATE_IS_NETWORK, 0],
                                   true, false);
     }
 
     // If the new tab is not busy, and our current state is busy, then
     // we need to fire a stop to all progress listeners.
-    if (!newTab.hasAttribute("busy") && this.mIsBusy) {
-      this.mIsBusy = false;
+    if (!newTab.hasAttribute("busy") && this._isBusy) {
+      this._isBusy = false;
       this._callProgressListeners(null, "onStateChange",
                                   [webProgress, null,
                                   nsIWebProgressListener.STATE_STOP |
                                   nsIWebProgressListener.STATE_IS_NETWORK, 0],
                                   true, false);
     }
 
     // TabSelect events are suppressed during preview mode to avoid confusing extensions and other bits of code
@@ -3093,17 +3093,17 @@ window._gBrowser = {
       // Icon might have been set on DOMLinkAdded, don't override that.
       if (!ourBrowser.mIconURL && otherBrowser.mIconURL)
         this.setIcon(aOurTab, otherBrowser.mIconURL, otherBrowser.contentPrincipal, otherBrowser.contentRequestContextID);
       var isBusy = aOtherTab.hasAttribute("busy");
       if (isBusy) {
         aOurTab.setAttribute("busy", "true");
         modifiedAttrs.push("busy");
         if (aOurTab.selected)
-          this.mIsBusy = true;
+          this._isBusy = true;
       }
 
       this._swapBrowserDocShells(aOurTab, otherBrowser, Ci.nsIBrowser.SWAP_DEFAULT, stateFlags);
     }
 
     // Unregister the previously opened URI
     if (otherBrowser.registeredOpenURI) {
       this._unifiedComplete.unregisterOpenPage(otherBrowser.registeredOpenURI,
@@ -4385,17 +4385,17 @@ class TabProgressListener {
               }
               return { continueMonitoring };
             },
           });
           gBrowser.syncThrobberAnimations(this.mTab);
         }
 
         if (this.mTab.selected) {
-          gBrowser.mIsBusy = true;
+          gBrowser._isBusy = true;
         }
       }
     } else if (aStateFlags & nsIWebProgressListener.STATE_STOP &&
                aStateFlags & nsIWebProgressListener.STATE_IS_NETWORK) {
 
       if (this.mTab.hasAttribute("busy")) {
         this.mTab.removeAttribute("busy");
         if (!document.querySelector(".tabbrowser-tab[busy]")) {
@@ -4456,17 +4456,17 @@ class TabProgressListener {
         }
       }
 
       // For keyword URIs clear the user typed value since they will be changed into real URIs
       if (location.scheme == "keyword")
         this.mBrowser.userTypedValue = null;
 
       if (this.mTab.selected)
-        gBrowser.mIsBusy = false;
+        gBrowser._isBusy = false;
     }
 
     if (ignoreBlank) {
       this._callProgressListeners("onUpdateCurrentBrowser",
                                   [aStateFlags, aStatus, "", 0],
                                   true, false);
     } else {
       this._callProgressListeners("onStateChange",