--- a/modules/libjar/nsJARURI.cpp
+++ b/modules/libjar/nsJARURI.cpp
@@ -264,43 +264,43 @@ nsJARURI::Mutator::SetFileName(const nsA
 {
     if (!mURI) {
         return NS_ERROR_NULL_POINTER;
     }
     if (aMutator) {
         nsCOMPtr<nsIURIMutator> mutator = this;
         mutator.forget(aMutator);
     }
-    return mURI->SetFileName(aFileName);
+    return mURI->SetFileNameInternal(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);
+    return mURI->SetFileBaseNameInternal(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);
+    return mURI->SetFileExtensionInternal(aFileExtension);
 }
 
 NS_IMETHODIMP
 nsJARURI::Mutate(nsIURIMutator** aMutator)
 {
     RefPtr<nsJARURI::Mutator> mutator = new nsJARURI::Mutator();
     nsresult rv = mutator->InitFromURI(this);
     if (NS_FAILED(rv)) {
@@ -702,62 +702,55 @@ nsJARURI::SetRef(const nsACString& ref)
 
 NS_IMETHODIMP
 nsJARURI::GetDirectory(nsACString& directory)
 {
     return mJAREntry->GetDirectory(directory);
 }
 
 NS_IMETHODIMP
-nsJARURI::SetDirectory(const nsACString& 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)
+nsresult
+nsJARURI::SetFileNameInternal(const nsACString& 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)
+nsresult
+nsJARURI::SetFileBaseNameInternal(const nsACString& 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)
+nsresult
+nsJARURI::SetFileExtensionInternal(const nsACString& fileExtension)
 {
     return NS_MutateURI(mJAREntry)
              .Apply<nsIURLMutator>(&nsIURLMutator::SetFileExtension,
                                    nsCString(fileExtension),
                                    nullptr)
              .Finalize(mJAREntry);
 }
 

--- a/modules/libjar/nsJARURI.h
+++ b/modules/libjar/nsJARURI.h
@@ -75,16 +75,20 @@ protected:
 
     // enum used in a few places to specify how .ref attribute should be handled
     enum RefHandlingEnum {
         eIgnoreRef,
         eHonorRef,
         eReplaceRef
     };
 
+    nsresult SetFileNameInternal(const nsACString& fileName);
+    nsresult SetFileBaseNameInternal(const nsACString& fileBaseName);
+    nsresult SetFileExtensionInternal(const nsACString& fileExtension);
+
     // Helper to share code between Equals methods.
     virtual nsresult EqualsInternal(nsIURI* other,
                                     RefHandlingEnum refHandlingMode,
                                     bool* result);
 
     // Helpers to share code between Clone methods.
     nsresult CloneWithJARFileInternal(nsIURI *jarFile,
                                       RefHandlingEnum refHandlingMode,

--- a/netwerk/base/nsIURL.idl
+++ b/netwerk/base/nsIURL.idl
@@ -43,48 +43,47 @@ interface nsIURL : nsIURI
      * the empty string. If the trailing slash is omitted, then the Directory
      * is /foo/ and the file is bar (i.e. this is a syntactic, not a semantic
      * breakdown of the Path).  And hence don't rely on this for something to
      * be a definitely be a file. But you can get just the leading directory
      * portion for sure.
      *
      * Some characters may be escaped.
      */
-    attribute AUTF8String directory;
+    readonly attribute AUTF8String directory;
 
     /**
      * Returns the file name portion of a URL.  If the URL denotes a path to a
      * directory and not a file, e.g. http://host/foo/bar/, then the Directory
      * attribute accesses the complete /foo/bar/ portion, and the FileName is
      * the empty string. Note that this is purely based on searching for the
      * last trailing slash. And hence don't rely on this to be a definite file. 
      *
      * Some characters may be escaped.
      */
-    attribute AUTF8String fileName;
-
+    readonly attribute AUTF8String fileName;
 
     /*************************************************************************
      * The URL filename is broken down even further:
      */
 
     /**
      * Returns the file basename portion of a filename in a url.
      *
      * Some characters may be escaped.
      */
-    attribute AUTF8String fileBaseName;
+    readonly attribute AUTF8String fileBaseName;
 
     /**
      * Returns the file extension portion of a filename in a url.  If a file
      * extension does not exist, the empty string is returned.
      *
      * Some characters may be escaped.
      */
-    attribute AUTF8String fileExtension;
+    readonly attribute AUTF8String fileExtension;
 
     /**
      * This method takes a uri and compares the two.  The common uri portion
      * is returned as a string.  The minimum common uri portion is the 
      * protocol, and any of these if present:  login, password, host and port
      * If no commonality is found, "" is returned.  If they are identical, the
      * whole path with file/ref/etc. is returned.  For file uris, it is
      * expected that the common spec would be at least "file:///" since '/' is

--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -3065,32 +3065,25 @@ nsStandardURL::SetRef(const nsACString &
     }
 
     int32_t shift = ReplaceSegment(mRef.mPos, mRef.mLen, ref, refLen);
     mPath.mLen += shift;
     mRef.mLen = refLen;
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsStandardURL::SetDirectory(const nsACString &input)
-{
-    MOZ_ASSERT_UNREACHABLE("SetDirectory");
-    return NS_ERROR_NOT_IMPLEMENTED;
-}
-
-NS_IMETHODIMP
-nsStandardURL::SetFileName(const nsACString &input)
+nsresult
+nsStandardURL::SetFileNameInternal(const nsACString &input)
 {
     ENSURE_MUTABLE();
 
     const nsPromiseFlatCString &flat = PromiseFlatCString(input);
     const char *filename = flat.get();
 
-    LOG(("nsStandardURL::SetFileName [filename=%s]\n", filename));
+    LOG(("nsStandardURL::SetFileNameInternal [filename=%s]\n", filename));
 
     if (mPath.mLen < 0)
         return SetPathQueryRef(flat);
 
     if (mSpec.Length() + input.Length() - Filename().Length() > (uint32_t) net_GetURLMaxLength()) {
         return NS_ERROR_MALFORMED_URI;
     }
 
@@ -3171,46 +3164,46 @@ nsStandardURL::SetFileName(const nsACStr
     if (shift) {
         ShiftFromQuery(shift);
         mFilepath.mLen += shift;
         mPath.mLen += shift;
     }
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsStandardURL::SetFileBaseName(const nsACString &input)
+nsresult
+nsStandardURL::SetFileBaseNameInternal(const nsACString &input)
 {
     nsAutoCString extension;
     nsresult rv = GetFileExtension(extension);
     NS_ENSURE_SUCCESS(rv, rv);
 
     nsAutoCString newFileName(input);
 
     if (!extension.IsEmpty()) {
         newFileName.Append('.');
         newFileName.Append(extension);
     }
 
-    return SetFileName(newFileName);
+    return SetFileNameInternal(newFileName);
 }
 
-NS_IMETHODIMP
-nsStandardURL::SetFileExtension(const nsACString &input)
+nsresult
+nsStandardURL::SetFileExtensionInternal(const nsACString &input)
 {
     nsAutoCString newFileName;
     nsresult rv = GetFileBaseName(newFileName);
     NS_ENSURE_SUCCESS(rv, rv);
 
     if (!input.IsEmpty()) {
         newFileName.Append('.');
         newFileName.Append(input);
     }
 
-    return SetFileName(newFileName);
+    return SetFileNameInternal(newFileName);
 }
 
 //----------------------------------------------------------------------------
 // nsStandardURL::nsIFileURL
 //----------------------------------------------------------------------------
 
 nsresult
 nsStandardURL::EnsureFile()

--- a/netwerk/base/nsStandardURL.h
+++ b/netwerk/base/nsStandardURL.h
@@ -180,16 +180,20 @@ protected:
     // returns NS_ERROR_NO_INTERFACE if the url does not map to a file
     virtual nsresult EnsureFile();
 
 private:
     nsresult Init(uint32_t urlType, int32_t defaultPort, const nsACString &spec,
                   const char *charset, nsIURI *baseURI);
     nsresult SetDefaultPort(int32_t aNewDefaultPort);
 
+    nsresult SetFileNameInternal(const nsACString &input);
+    nsresult SetFileBaseNameInternal(const nsACString &input);
+    nsresult SetFileExtensionInternal(const nsACString &input);
+
     int32_t  Port() { return mPort == -1 ? mDefaultPort : mPort; }
 
     void     ReplacePortInSpec(int32_t aNewPort);
     void     Clear();
     void     InvalidateCache(bool invalidateCachedFile = true);
 
     bool     ValidIPv6orHostname(const char *host, uint32_t aLen);
     static bool     IsValidOfBase(unsigned char c, const uint32_t base);
@@ -401,43 +405,43 @@ public:
         {
             if (!BaseURIMutator<T>::mURI) {
                 return NS_ERROR_NULL_POINTER;
             }
             if (aMutator) {
                 nsCOMPtr<nsIURIMutator> mutator = this;
                 mutator.forget(aMutator);
             }
-            return BaseURIMutator<T>::mURI->SetFileName(aFileName);
+            return BaseURIMutator<T>::mURI->SetFileNameInternal(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);
+            return BaseURIMutator<T>::mURI->SetFileBaseNameInternal(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);
+            return BaseURIMutator<T>::mURI->SetFileExtensionInternal(aFileExtension);
         }
 
         MOZ_MUST_USE NS_IMETHOD
         SetFile(nsIFile* aFile) override
         {
             RefPtr<T> uri;
             if (BaseURIMutator<T>::mURI) {
                 // We don't need a new URI object if we already have one
@@ -445,17 +449,17 @@ public:
             } else {
                 uri = new T(/* supportsFileURL = */ true);
             }
 
             nsresult rv = uri->SetFile(aFile);
             if (NS_FAILED(rv)) {
                 return rv;
             }
-            BaseURIMutator<T>::mURI = uri;
+            BaseURIMutator<T>::mURI.swap(uri);
             return NS_OK;
         }
 
         explicit TemplatedMutator() { }
     private:
         virtual ~TemplatedMutator() { }
 
         friend T;