diff --git a/modules/libjar/nsIJARURI.idl b/modules/libjar/nsIJARURI.idl
index bbbfe9c45418..3b61c610345b 100644
--- a/modules/libjar/nsIJARURI.idl
+++ b/modules/libjar/nsIJARURI.idl
@@ -36,3 +36,12 @@ interface nsIJARURI : nsIURL {
      */
     nsIJARURI cloneWithJARFile(in nsIURI jarFile);
 };
+
+[builtinclass, uuid(d66df117-eda7-4324-b4e4-1f670ff6718e)]
+interface nsIJARURIMutator : nsISupports
+{
+    /**
+     * Will initalize a URI using the passed spec, baseURI and charset.
+     */
+    void setSpecBaseCharset(in AUTF8String aSpec, in nsIURI aBase, in string aCharset);
+};
diff --git a/modules/libjar/nsJARProtocolHandler.cpp b/modules/libjar/nsJARProtocolHandler.cpp
index f6f3e47deed6..41bfcb7489e1 100644
--- a/modules/libjar/nsJARProtocolHandler.cpp
+++ b/modules/libjar/nsJARProtocolHandler.cpp
@@ -118,21 +118,11 @@ nsJARProtocolHandler::NewURI(const nsACString &aSpec,
                              nsIURI *aBaseURI,
                              nsIURI **result)
 {
-    nsresult rv = NS_OK;
-
-    RefPtr<nsJARURI> jarURI = new nsJARURI();
-    if (!jarURI)
-        return NS_ERROR_OUT_OF_MEMORY;
-
-    rv = jarURI->Init(aCharset);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    rv = jarURI->SetSpecWithBase(aSpec, aBaseURI);
-    if (NS_FAILED(rv))
-        return rv;
-
-    NS_ADDREF(*result = jarURI);
-    return rv;
+    nsCOMPtr<nsIURI> base(aBaseURI);
+    return NS_MutateURI(new nsJARURI::Mutator())
+             .Apply(NS_MutatorMethod(&nsIJARURIMutator::SetSpecBaseCharset,
+                                     nsCString(aSpec), base, aCharset))
+             .Finalize(result);
 }
 
 NS_IMETHODIMP
diff --git a/modules/libjar/nsJARURI.cpp b/modules/libjar/nsJARURI.cpp
index 242854d4b48d..0382cf51937c 100644
--- a/modules/libjar/nsJARURI.cpp
+++ b/modules/libjar/nsJARURI.cpp
@@ -268,7 +268,8 @@ NS_IMPL_NSIURIMUTATOR_ISUPPORTS(nsJARURI::Mutator,
                                 nsIURISetters,
                                 nsIURIMutator,
                                 nsIURLMutator,
-                                nsISerializable)
+                                nsISerializable,
+                                nsIJARURIMutator)
 
 NS_IMETHODIMP
 nsJARURI::Mutator::SetFileName(const nsACString& aFileName, nsIURIMutator** aMutator)
diff --git a/modules/libjar/nsJARURI.h b/modules/libjar/nsJARURI.h
index 8669bf13ac9f..942f7cd493db 100644
--- a/modules/libjar/nsJARURI.h
+++ b/modules/libjar/nsJARURI.h
@@ -59,19 +59,18 @@ public:
     NS_DECLARE_STATIC_IID_ACCESSOR(NS_THIS_JARURI_IMPL_CID)
 
     // nsJARURI
-    nsJARURI();
-
-    nsresult Init(const char *charsetHint);
     nsresult FormatSpec(const nsACString &entryPath, nsACString &result,
                         bool aIncludeScheme = true);
     nsresult CreateEntryURL(const nsACString& entryFilename,
                             const char* charset,
                             nsIURL** url);
-    nsresult SetSpecWithBase(const nsACString& aSpec, nsIURI* aBaseURL);
 
 protected:
+    nsJARURI();
     virtual ~nsJARURI();
     nsresult SetJAREntry(const nsACString &entryPath);
+    nsresult Init(const char *charsetHint);
+    nsresult SetSpecWithBase(const nsACString& aSpec, nsIURI* aBaseURL);
 
     // enum used in a few places to specify how .ref attribute should be handled
     enum RefHandlingEnum {
@@ -126,6 +125,7 @@ public:
         , public BaseURIMutator<nsJARURI>
         , public nsIURLMutator
         , public nsISerializable
+        , public nsIJARURIMutator
     {
         NS_DECL_ISUPPORTS
         NS_FORWARD_SAFE_NSIURISETTERS_RET(mURI)
@@ -144,6 +144,30 @@ public:
             return InitFromInputStream(aStream);
         }
 
+        NS_IMETHOD
+        SetSpecBaseCharset(const nsACString& aSpec,
+                           nsIURI* aBaseURI,
+                           const char* aCharset) override
+        {
+            RefPtr<nsJARURI> uri;
+            if (mURI) {
+                mURI.swap(uri);
+            } else {
+                uri = new nsJARURI();
+            }
+
+            nsresult rv = uri->Init(aCharset);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            rv = uri->SetSpecWithBase(aSpec, aBaseURI);
+            if (NS_FAILED(rv)) {
+                return rv;
+            }
+
+            mURI.swap(uri);
+            return NS_OK;
+        }
+
         explicit Mutator() { }
     private:
         virtual ~Mutator() { }