--- a/dom/base/Link.cpp
+++ b/dom/base/Link.cpp
@@ -457,24 +457,28 @@ Link::SetPassword(const nsAString &aPass
 
   uri->SetPassword(NS_ConvertUTF16toUTF8(aPassword));
   SetHrefAttribute(uri);
 }
 
 void
 Link::SetUsername(const nsAString &aUsername)
 {
-  nsCOMPtr<nsIURI> uri(GetURIToMutate());
+  nsCOMPtr<nsIURI> uri(GetURI());
   if (!uri) {
     // Ignore failures to be compatible with NS4.
     return;
   }
 
-  uri->SetUsername(NS_ConvertUTF16toUTF8(aUsername));
-  SetHrefAttribute(uri);
+  nsresult rv = NS_MutateURI(uri)
+                  .SetUsername(NS_ConvertUTF16toUTF8(aUsername))
+                  .Finalize(uri);
+  if (NS_SUCCEEDED(rv)) {
+    SetHrefAttribute(uri);
+  }
 }
 
 void
 Link::SetHost(const nsAString &aHost)
 {
   nsCOMPtr<nsIURI> uri(GetURI());
   if (!uri) {
     // Ignore failures to be compatible with NS4.

--- a/dom/url/URLMainThread.cpp
+++ b/dom/url/URLMainThread.cpp
@@ -287,17 +287,19 @@ void
 URLMainThread::GetUsername(nsAString& aUsername, ErrorResult& aRv) const
 {
   URL_GETTER(aUsername, GetUsername);
 }
 
 void
 URLMainThread::SetUsername(const nsAString& aUsername, ErrorResult& aRv)
 {
-  mURI->SetUsername(NS_ConvertUTF16toUTF8(aUsername));
+  Unused << NS_MutateURI(mURI)
+              .SetUsername(NS_ConvertUTF16toUTF8(aUsername))
+              .Finalize(mURI);
 }
 
 void
 URLMainThread::GetPassword(nsAString& aPassword, ErrorResult& aRv) const
 {
   URL_GETTER(aPassword, GetPassword);
 }
 

--- a/netwerk/test/unit/test_URIs.js
+++ b/netwerk/test/unit/test_URIs.js
@@ -519,17 +519,17 @@ function do_test_mutate_ref(aTest, aSuff
 
 // Tests that normally-mutable properties can't be modified on
 // special URIs that are known to be immutable.
 function do_test_immutable(aTest) {
   Assert.ok(aTest.immutable);
 
   var URI = NetUtil.newURI(aTest.spec);
   // All the non-readonly attributes on nsIURI.idl:
-  var propertiesToCheck = ["spec", "scheme", "username", "password",
+  var propertiesToCheck = ["spec", "scheme", "password",
                            "host", "port", "query", "ref"];
 
   propertiesToCheck.forEach(function(aProperty) {
     var threw = false;
     try {
       URI[aProperty] = "anothervalue";
     } catch(e) {
       threw = true;

--- a/netwerk/test/unit/test_URIs2.js
+++ b/netwerk/test/unit/test_URIs2.js
@@ -620,17 +620,17 @@ function do_test_mutate_ref(aTest, aSuff
 
 // Tests that normally-mutable properties can't be modified on
 // special URIs that are known to be immutable.
 function do_test_immutable(aTest) {
   Assert.ok(aTest.immutable);
 
   var URI = NetUtil.newURI(aTest.spec);
   // All the non-readonly attributes on nsIURI.idl:
-  var propertiesToCheck = ["scheme", "username", "password",
+  var propertiesToCheck = ["scheme", "password",
                            "host", "port", "query", "ref"];
 
   propertiesToCheck.forEach(function(aProperty) {
     var threw = false;
     try {
       URI[aProperty] = "anothervalue";
     } catch(e) {
       threw = true;

--- a/netwerk/test/unit/test_standardurl.js
+++ b/netwerk/test/unit/test_standardurl.js
@@ -296,27 +296,28 @@ add_test(function test_percentDecoding()
 add_test(function test_hugeStringThrows()
 {
   let prefs = Cc["@mozilla.org/preferences-service;1"]
                 .getService(Ci.nsIPrefService);
   let maxLen = prefs.getIntPref("network.standard-url.max-length");
   let url = stringToURL("http://test:test@example.com");
 
   let hugeString = new Array(maxLen + 1).fill("a").join("");
-  let properties = ["scheme", "username",
+  let properties = ["scheme",
                     "password", "host", "ref",
                     "query"];
   for (let prop of properties) {
     Assert.throws(() => url[prop] = hugeString,
                   /NS_ERROR_MALFORMED_URI/,
                   `Passing a huge string to "${prop}" should throw`);
   }
 
   let setters = [
     { method: "setSpec", qi: Ci.nsIURIMutator },
+    { method: "setUsername", qi: Ci.nsIURIMutator },
     { method: "setFilePath", qi: Ci.nsIURIMutator },
     { method: "setHostPort", qi: Ci.nsIURIMutator },
     { method: "setUserPass", qi: Ci.nsIURIMutator },
     { method: "setPathQueryRef", qi: Ci.nsIURIMutator },
     { method: "setFileName", qi: Ci.nsIURLMutator },
     { method: "setFileExtension", qi: Ci.nsIURLMutator },
     { method: "setFileBaseName", qi: Ci.nsIURLMutator },
   ];