diff --git a/toolkit/modules/RemoteWebProgress.jsm b/toolkit/modules/RemoteWebProgress.jsm
index 90449d0ddc4c..b6c48533adf0 100644
--- a/toolkit/modules/RemoteWebProgress.jsm
+++ b/toolkit/modules/RemoteWebProgress.jsm
@@ -222,7 +222,11 @@ RemoteWebProgressManager.prototype = {
 
     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;
       }