diff --git a/toolkit/components/passwordmgr/LoginManagerPrompter.jsm b/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
index a9ab9d5fa3df..fd06fba4199f 100644
--- a/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerPrompter.jsm
@@ -959,12 +959,14 @@ LoginManagerPrompter.prototype = {
    *        Options to pass along to PopupNotifications.show().
    * @param {bool} [options.notifySaved = false]
    *        Whether to indicate to the user that the login was already saved.
+   * @param {string} [options.messageStringID = undefined]
+   *        An optional string ID to override the default message.
    */
   _showLoginCaptureDoorhanger(
     login,
     type,
     showOptions = {},
-    { notifySaved = false } = {}
+    { notifySaved = false, messageStringID } = {}
   ) {
     let { browser } = this._getNotifyWindow();
     if (!browser) {
@@ -990,6 +992,10 @@ LoginManagerPrompter.prototype = {
     let initialMsgNames =
       type == "password-save" ? saveMsgNames : changeMsgNames;
 
+    if (messageStringID) {
+      changeMsgNames.prompt = messageStringID;
+    }
+
     let brandBundle = Services.strings.createBundle(BRAND_BUNDLE);
     let brandShortName = brandBundle.GetStringFromName("brandShortName");
     let host = this._getShortDisplayHost(login.origin);
@@ -1452,6 +1458,19 @@ LoginManagerPrompter.prototype = {
     login.formActionOrigin = aNewLogin.formActionOrigin;
     login.password = aNewLogin.password;
     login.username = aNewLogin.username;
+
+    let messageStringID;
+    if (
+      aOldLogin.username === "" &&
+      login.username !== "" &&
+      login.password == aOldLogin.password
+    ) {
+      // If the saved password matches the password we're prompting with then we
+      // are only prompting to let the user add a username since there was one in
+      // the form. Change the message so the purpose of the prompt is clearer.
+      messageStringID = "updateLoginMsgAddUsername";
+    }
+
     this._showLoginCaptureDoorhanger(
       login,
       "password-change",
@@ -1461,6 +1480,7 @@ LoginManagerPrompter.prototype = {
       },
       {
         notifySaved,
+        messageStringID,
       }
     );
 
diff --git a/toolkit/components/passwordmgr/test/browser/browser_capture_doorhanger.js b/toolkit/components/passwordmgr/test/browser/browser_capture_doorhanger.js
index 9aba647bcbe8..30e5214f6b09 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_capture_doorhanger.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_capture_doorhanger.js
@@ -314,6 +314,11 @@ add_task(async function test_pwOnlyLoginMatchesForm() {
     is(fieldValues.password, "notifyp1", "Checking submitted password");
     let notif = getCaptureDoorhanger("password-change");
     ok(notif, "checking for notification popup");
+    is(
+      notif.message,
+      "Would you like to add a username to the saved password?",
+      "Check message"
+    );
     notif.remove();
   });
 
@@ -389,6 +394,7 @@ add_task(async function test_changeUPLoginOnUPForm_dont() {
     is(fieldValues.password, "pass2", "Checking submitted password");
     let notif = getCaptureDoorhanger("password-change");
     ok(notif, "got notification popup");
+    is(notif.message, "Would you like to update this login?", "Check message");
 
     await checkDoorhangerUsernamePassword("notifyu1", "pass2");
     clickDoorhangerButton(notif, DONT_CHANGE_BUTTON);
@@ -415,6 +421,7 @@ add_task(async function test_changeUPLoginOnUPForm_change() {
     is(fieldValues.password, "pass2", "Checking submitted password");
     let notif = getCaptureDoorhanger("password-change");
     ok(notif, "got notification popup");
+    is(notif.message, "Would you like to update this login?", "Check message");
 
     await checkDoorhangerUsernamePassword("notifyu1", "pass2");
     clickDoorhangerButton(notif, CHANGE_BUTTON);
@@ -438,7 +445,7 @@ add_task(async function test_changeUPLoginOnUPForm_change() {
 });
 
 add_task(async function test_changePLoginOnUPForm() {
-  info("Check for change-password popup, p-only login on u+p form.");
+  info("Check for change-password popup, p-only login on u+p form (empty u).");
   Services.logins.addLogin(login2);
 
   await testSubmittingLoginForm("subtst_notifications_9.html", async function(
@@ -448,6 +455,7 @@ add_task(async function test_changePLoginOnUPForm() {
     is(fieldValues.password, "pass2", "Checking submitted password");
     let notif = getCaptureDoorhanger("password-change");
     ok(notif, "got notification popup");
+    is(notif.message, "Would you like to update this password?", "Check msg");
 
     await checkDoorhangerUsernamePassword("", "pass2");
     clickDoorhangerButton(notif, CHANGE_BUTTON);
@@ -475,6 +483,7 @@ add_task(async function test_changePLoginOnPForm() {
     is(fieldValues.password, "notifyp1", "Checking submitted password");
     let notif = getCaptureDoorhanger("password-change");
     ok(notif, "got notification popup");
+    is(notif.message, "Would you like to update this password?", "Check msg");
 
     await checkDoorhangerUsernamePassword("", "notifyp1");
     clickDoorhangerButton(notif, CHANGE_BUTTON);
diff --git a/toolkit/locales/en-US/chrome/passwordmgr/passwordmgr.properties b/toolkit/locales/en-US/chrome/passwordmgr/passwordmgr.properties
index 759cb87f75f0..83ac437a4041 100644
--- a/toolkit/locales/en-US/chrome/passwordmgr/passwordmgr.properties
+++ b/toolkit/locales/en-US/chrome/passwordmgr/passwordmgr.properties
@@ -17,6 +17,7 @@ saveLoginButtonNever.label = Never Save
 saveLoginButtonNever.accesskey = e
 updateLoginMsg = Would you like to update this login?
 updateLoginMsgNoUser = Would you like to update this password?
+updateLoginMsgAddUsername = Would you like to add a username to the saved password?
 updateLoginButtonText = Update
 updateLoginButtonAccessKey = U
 updateLoginButtonDeny.label = Don’t Update