--- a/modules/libjar/nsJARURI.cpp
+++ b/modules/libjar/nsJARURI.cpp
@@ -252,17 +252,56 @@ nsJARURI::GetHasRef(bool *result)
 }
 
 nsresult
 nsJARURI::SetSpecInternal(const nsACString& aSpec)
 {
     return SetSpecWithBase(aSpec, nullptr);
 }
 
-NS_IMPL_ISUPPORTS(nsJARURI::Mutator, nsIURISetters, nsIURIMutator)
+NS_IMPL_ISUPPORTS(nsJARURI::Mutator, nsIURISetters, nsIURIMutator, nsIURLMutator)
+
+NS_IMETHODIMP
+nsJARURI::Mutator::SetFileName(const nsACString& aFileName, nsIURIMutator** aMutator)
+{
+    if (!mURI) {
+        return NS_ERROR_NULL_POINTER;
+    }
+    if (aMutator) {
+        nsCOMPtr<nsIURIMutator> mutator = this;
+        mutator.forget(aMutator);
+    }
+    return mURI->SetFileName(aFileName);
+}
+
+NS_IMETHODIMP
+nsJARURI::Mutator::SetFileBaseName(const nsACString& aFileBaseName, nsIURIMutator** aMutator)
+{
+    if (!mURI) {
+        return NS_ERROR_NULL_POINTER;
+    }
+    if (aMutator) {
+        nsCOMPtr<nsIURIMutator> mutator = this;
+        mutator.forget(aMutator);
+    }
+    return mURI->SetFileBaseName(aFileBaseName);
+}
+
+NS_IMETHODIMP
+nsJARURI::Mutator::SetFileExtension(const nsACString& aFileExtension, nsIURIMutator** aMutator)
+{
+    if (!mURI) {
+        return NS_ERROR_NULL_POINTER;
+    }
+    if (aMutator) {
+        nsCOMPtr<nsIURIMutator> mutator = this;
+        mutator.forget(aMutator);
+    }
+    return mURI->SetFileExtension(aFileExtension);
+}
 
 NS_IMETHODIMP
 nsJARURI::Mutate(nsIURIMutator** aMutator)
 {
     RefPtr<nsJARURI::Mutator> mutator = new nsJARURI::Mutator();
     nsresult rv = mutator->InitFromURI(this);
     if (NS_FAILED(rv)) {
         return rv;
@@ -665,53 +704,66 @@ NS_IMETHODIMP
 nsJARURI::GetDirectory(nsACString& directory)
 {
     return mJAREntry->GetDirectory(directory);
 }
 
 NS_IMETHODIMP
 nsJARURI::SetDirectory(const nsACString& directory)
 {
-    return mJAREntry->SetDirectory(directory);
+    MOZ_ASSERT_UNREACHABLE("SetDirectory");
+    return NS_ERROR_NOT_IMPLEMENTED;
 }
 
 NS_IMETHODIMP
 nsJARURI::GetFileName(nsACString& fileName)
 {
     return mJAREntry->GetFileName(fileName);
 }
 
 NS_IMETHODIMP
 nsJARURI::SetFileName(const nsACString& fileName)
 {
-    return mJAREntry->SetFileName(fileName);
+    return NS_MutateURI(mJAREntry)
+             .Apply<nsIURLMutator>(&nsIURLMutator::SetFileName,
+                                   nsCString(fileName),
+                                   nullptr)
+             .Finalize(mJAREntry);
 }
 
 NS_IMETHODIMP
 nsJARURI::GetFileBaseName(nsACString& fileBaseName)
 {
     return mJAREntry->GetFileBaseName(fileBaseName);
 }
 
 NS_IMETHODIMP
 nsJARURI::SetFileBaseName(const nsACString& fileBaseName)
 {
-    return mJAREntry->SetFileBaseName(fileBaseName);
+    return NS_MutateURI(mJAREntry)
+             .Apply<nsIURLMutator>(&nsIURLMutator::SetFileBaseName,
+                                   nsCString(fileBaseName),
+                                   nullptr)
+             .Finalize(mJAREntry);
 }
 
 NS_IMETHODIMP
 nsJARURI::GetFileExtension(nsACString& fileExtension)
 {
     return mJAREntry->GetFileExtension(fileExtension);
 }
 
 NS_IMETHODIMP
 nsJARURI::SetFileExtension(const nsACString& fileExtension)
 {
-    return mJAREntry->SetFileExtension(fileExtension);
+    return NS_MutateURI(mJAREntry)
+             .Apply<nsIURLMutator>(&nsIURLMutator::SetFileExtension,
+                                   nsCString(fileExtension),
+                                   nullptr)
+             .Finalize(mJAREntry);
 }
 
 NS_IMETHODIMP
 nsJARURI::GetCommonBaseSpec(nsIURI* uriToCompare, nsACString& commonSpec)
 {
     commonSpec.Truncate();
 
     NS_ENSURE_ARG_POINTER(uriToCompare);

--- a/modules/libjar/nsJARURI.h
+++ b/modules/libjar/nsJARURI.h
@@ -97,20 +97,22 @@ protected:
     // like extensions, refs, etc.
     nsCOMPtr<nsIURL> mJAREntry;
     nsCString        mCharsetHint;
 
 public:
     class Mutator
         : public nsIURIMutator
         , public BaseURIMutator<nsJARURI>
+        , public nsIURLMutator
     {
         NS_DECL_ISUPPORTS
         NS_FORWARD_SAFE_NSIURISETTERS_RET(mURI)
         NS_DEFINE_NSIMUTATOR_COMMON
+        NS_DECL_NSIURLMUTATOR
 
         explicit Mutator() { }
     private:
         virtual ~Mutator() { }
 
         friend class nsJARURI;
     };
 };

--- a/netwerk/base/nsIURL.idl
+++ b/netwerk/base/nsIURL.idl
@@ -1,14 +1,15 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsIURI.idl"
+interface nsIURIMutator;
 
 /**
  * The nsIURL interface provides convenience methods that further
  * break down the path portion of nsIURI:
  *
  * http://host/directory/fileBaseName.fileExtension?query
  * http://host/directory/fileBaseName.fileExtension#ref
  *            \          \                       /
@@ -115,8 +116,16 @@ interface nsIURL : nsIURI
      * 2) http://mozilla.org/     http://www.mozilla.org    http://www.mozilla.org/
      * 3) http://foo.com/bar/     http://foo.com:80/bar/    ""
      * 4) http://foo.com/         http://foo.com/a.htm#b    a.html#b
      * 5) http://foo.com/a/b/     http://foo.com/c          ../../c
      */
     AUTF8String getRelativeSpec(in nsIURI aURIToCompare);
 
 };
+
+[scriptable, builtinclass, uuid(25072eb8-f1e6-482f-9ca9-eddd3d65169a)]
+interface nsIURLMutator : nsISupports
+{
+    [must_use] nsIURIMutator setFileName(in AUTF8String aFileName);
+    [must_use] nsIURIMutator setFileBaseName(in AUTF8String aFileBaseName);
+    [must_use] nsIURIMutator setFileExtension(in AUTF8String aFileExtension);
+};

--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -2221,17 +2221,18 @@ nsStandardURL::SetPathQueryRef(const nsA
         mBasename.mLen = -1;
         mExtension.mLen = -1;
         mQuery.mLen = -1;
         mRef.mLen = -1;
     }
     return NS_OK;
 }
 
-NS_IMPL_ISUPPORTS(nsStandardURL::Mutator, nsIURISetters, nsIURIMutator, nsIStandardURLMutator)
+// When updating this also update SubstitutingURL::Mutator
+NS_IMPL_ISUPPORTS(nsStandardURL::Mutator, nsIURISetters, nsIURIMutator, nsIStandardURLMutator, nsIURLMutator)
 
 NS_IMETHODIMP
 nsStandardURL::Mutate(nsIURIMutator** aMutator)
 {
     RefPtr<nsStandardURL::Mutator> mutator = new nsStandardURL::Mutator();
     nsresult rv = mutator->InitFromURI(this);
     if (NS_FAILED(rv)) {
         return rv;

--- a/netwerk/base/nsStandardURL.h
+++ b/netwerk/base/nsStandardURL.h
@@ -316,16 +316,17 @@ public:
 
     // We make this implementation a template so that we can avoid writing
     // the same code for SubstitutingURL (which extends nsStandardURL)
     template<class T>
     class TemplatedMutator
         : public nsIURIMutator
         , public BaseURIMutator<T>
         , public nsIStandardURLMutator
+        , public nsIURLMutator
     {
         NS_FORWARD_SAFE_NSIURISETTERS_RET(BaseURIMutator<T>::mURI)
 
         MOZ_MUST_USE NS_IMETHOD
         Deserialize(const mozilla::ipc::URIParams& aParams) override
         {
             return BaseURIMutator<T>::InitFromIPCParams(aParams);
         }
@@ -385,16 +386,56 @@ public:
             }
             if (aMutator) {
                 nsCOMPtr<nsIURIMutator> mutator = this;
                 mutator.forget(aMutator);
             }
             return BaseURIMutator<T>::mURI->SetDefaultPort(aNewDefaultPort);
         }
 
+        MOZ_MUST_USE NS_IMETHOD
+        SetFileName(const nsACString& aFileName, nsIURIMutator** aMutator) override
+        {
+            if (!BaseURIMutator<T>::mURI) {
+                return NS_ERROR_NULL_POINTER;
+            }
+            if (aMutator) {
+                nsCOMPtr<nsIURIMutator> mutator = this;
+                mutator.forget(aMutator);
+            }
+            return BaseURIMutator<T>::mURI->SetFileName(aFileName);
+        }
+
+        MOZ_MUST_USE NS_IMETHOD
+        SetFileBaseName(const nsACString& aFileBaseName, nsIURIMutator** aMutator) override
+        {
+            if (!BaseURIMutator<T>::mURI) {
+                return NS_ERROR_NULL_POINTER;
+            }
+            if (aMutator) {
+                nsCOMPtr<nsIURIMutator> mutator = this;
+                mutator.forget(aMutator);
+            }
+            return BaseURIMutator<T>::mURI->SetFileBaseName(aFileBaseName);
+        }
+
+        MOZ_MUST_USE NS_IMETHOD
+        SetFileExtension(const nsACString& aFileExtension, nsIURIMutator** aMutator) override
+        {
+            if (!BaseURIMutator<T>::mURI) {
+                return NS_ERROR_NULL_POINTER;
+            }
+            if (aMutator) {
+                nsCOMPtr<nsIURIMutator> mutator = this;
+                mutator.forget(aMutator);
+            }
+            return BaseURIMutator<T>::mURI->SetFileExtension(aFileExtension);
+        }
+
+
         explicit TemplatedMutator() { }
     private:
         virtual ~TemplatedMutator() { }
 
         friend T;
     };
 
     class Mutator

--- a/netwerk/protocol/res/SubstitutingProtocolHandler.cpp
+++ b/netwerk/protocol/res/SubstitutingProtocolHandler.cpp
@@ -27,17 +27,17 @@ namespace net {
 static LazyLogModule gResLog("nsResProtocol");
 
 static NS_DEFINE_CID(kSubstitutingURLCID, NS_SUBSTITUTINGURL_CID);
 
 //---------------------------------------------------------------------------------
 // SubstitutingURL : overrides nsStandardURL::GetFile to provide nsIFile resolution
 //---------------------------------------------------------------------------------
 
-NS_IMPL_ISUPPORTS(SubstitutingURL::Mutator, nsIURISetters, nsIURIMutator, nsIStandardURLMutator)
+NS_IMPL_ISUPPORTS(SubstitutingURL::Mutator, nsIURISetters, nsIURIMutator, nsIStandardURLMutator, nsIURLMutator)
 
 nsresult
 SubstitutingURL::EnsureFile()
 {
   nsAutoCString ourScheme;
   nsresult rv = GetScheme(ourScheme);
   NS_ENSURE_SUCCESS(rv, rv);