--- a/dom/base/nsHostObjectURI.cpp
+++ b/dom/base/nsHostObjectURI.cpp
@@ -76,16 +76,25 @@ nsHostObjectURI::Write(nsIObjectOutputSt
   nsresult rv = nsSimpleURI::Write(aStream);
   NS_ENSURE_SUCCESS(rv, rv);
 
   return NS_WriteOptionalCompoundObject(aStream, mPrincipal,
                                         NS_GET_IID(nsIPrincipal),
                                         true);
 }
 
+NS_IMETHODIMP
+nsHostObjectURI::SetScheme(const nsACString& aScheme)
+{
+  // Disallow setting the scheme, since that could cause us to be associated
+  // with a different protocol handler that doesn't expect us to be carrying
+  // around a principal with nsIURIWithPrincipal.
+  return NS_ERROR_FAILURE;
+}
+
 // nsIURI methods:
 nsresult
 nsHostObjectURI::CloneInternal(nsSimpleURI::RefHandlingEnum aRefHandlingMode,
                                nsIURI** aClone)
 {
   nsCOMPtr<nsIURI> simpleClone;
   nsresult rv =
     nsSimpleURI::CloneInternal(aRefHandlingMode, getter_AddRefs(simpleClone));

--- a/dom/base/nsHostObjectURI.h
+++ b/dom/base/nsHostObjectURI.h
@@ -29,16 +29,18 @@ public:
   // For use only from deserialization
   nsHostObjectURI() : nsSimpleURI() {}
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_NSIURIWITHPRINCIPAL
   NS_DECL_NSISERIALIZABLE
   NS_DECL_NSICLASSINFO
 
+  NS_IMETHOD SetScheme(const nsACString &aProtocol) override;
+
   // Override CloneInternal() and EqualsInternal()
   virtual nsresult CloneInternal(RefHandlingEnum aRefHandlingMode,
                                  nsIURI** aClone) override;
   virtual nsresult EqualsInternal(nsIURI* aOther,
                                   RefHandlingEnum aRefHandlingMode,
                                   bool* aResult) override;
 
   // Override StartClone to hand back a nsHostObjectURI