--- a/browser/base/content/browser-fullZoom.js
+++ b/browser/base/content/browser-fullZoom.js
@@ -206,18 +206,22 @@ var FullZoom = {
 
     if (!aURI || (aIsTabSwitch && !this.siteSpecific)) {
       this._notifyOnLocationChange(browser);
       return;
     }
 
     // Avoid the cps roundtrip and apply the default/global pref.
     if (aURI.spec == "about:blank") {
-      this._applyPrefToZoom(undefined, browser,
-                            this._notifyOnLocationChange.bind(this, browser));
+      // If it's a null principal, it should be a blank page, so we don't need
+      // to do anything.
+      if (browser.contentPrincipal && !browser.contentPrincipal.isNullPrincipal) {
+        this._applyPrefToZoom(undefined, browser,
+                              this._notifyOnLocationChange.bind(this, browser));
+      }
       return;
     }
 
     // Media documents should always start at 1, and are not affected by prefs.
     if (!aIsTabSwitch && browser.isSyntheticDocument) {
       ZoomManager.setZoomForBrowser(browser, 1);
       // _ignorePendingZoomAccesses already called above, so no need here.
       this._notifyOnLocationChange(browser);