--- a/toolkit/modules/RemoteWebProgress.jsm
+++ b/toolkit/modules/RemoteWebProgress.jsm
@@ -217,17 +217,21 @@ RemoteWebProgressManager.prototype = {
     if (json.requestURI) {
       request = new RemoteWebProgressRequest(json.requestURI,
                                              json.originalRequestURI,
                                              objects.request);
     }
 
     if (isTopLevel) {
       this._browser._contentWindow = objects.contentWindow;
-      this._browser._documentContentType = json.documentContentType;
+      // Setting a content-type back to `null` is quite nonsensical for the
+      // frontend, especially since we're not expecting it.
+      if (json.documentContentType !== null) {
+        this._browser._documentContentType = json.documentContentType;
+      }
       if (typeof json.inLoadURI != "undefined") {
         this._browser.inLoadURI = json.inLoadURI;
       }
       if (json.charset) {
         this._browser._characterSet = json.charset;
         this._browser._mayEnableCharacterEncodingMenu = json.mayEnableCharacterEncodingMenu;
       }
     }