diff --git a/toolkit/components/passwordmgr/LoginManagerContent.jsm b/toolkit/components/passwordmgr/LoginManagerContent.jsm
index 7a469548897..a3f2878e90a 100644
--- a/toolkit/components/passwordmgr/LoginManagerContent.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerContent.jsm
@@ -713,18 +713,20 @@ var LoginManagerContent = {
                 // Don't modify the username field if it's disabled or readOnly so we preserve its case.
                 let disabledOrReadOnly = usernameField.disabled || usernameField.readOnly;
 
+                let userNameDiffers = selectedLogin.username != usernameField.value;
                 // Don't replace the username if it differs only in case, and the user triggered
                 // this autocomplete. We assume that if it was user-triggered the entered text
                 // is desired.
-                let userEnteredDifferentCase = userTriggered &&
-                      (usernameField.value != selectedLogin.username &&
-                       usernameField.value.toLowerCase() == selectedLogin.username.toLowerCase());
+                let userEnteredDifferentCase = userTriggered && userNameDiffers &&
+                       usernameField.value.toLowerCase() == selectedLogin.username.toLowerCase();
 
-                if (!disabledOrReadOnly && !userEnteredDifferentCase) {
+                if (!disabledOrReadOnly && !userEnteredDifferentCase && userNameDiffers) {
                     usernameField.setUserInput(selectedLogin.username);
                 }
             }
-            passwordField.setUserInput(selectedLogin.password);
+            if (passwordField.value != selectedLogin.password) {
+                passwordField.setUserInput(selectedLogin.password);
+            }
             didFillForm = true;
         } else if (selectedLogin && !autofillForm) {
             // For when autofillForm is false, but we still have the information
diff --git a/toolkit/components/passwordmgr/test/mochitest.ini b/toolkit/components/passwordmgr/test/mochitest.ini
index 4f8754b0553..5507e958419 100644
--- a/toolkit/components/passwordmgr/test/mochitest.ini
+++ b/toolkit/components/passwordmgr/test/mochitest.ini
@@ -61,6 +61,7 @@ skip-if = toolkit == 'android' #TIMED_OUT
 [test_bug_654348.html]
 [test_bug_776171.html]
 [test_input_events.html]
+[test_input_events_for_identical_values.html]
 [test_master_password.html]
 skip-if = toolkit == 'android' #TIMED_OUT
 [test_master_password_cleanup.html]
diff --git a/toolkit/components/passwordmgr/test/test_input_events_for_identical_values.html b/toolkit/components/passwordmgr/test/test_input_events_for_identical_values.html
new file mode 100644
index 00000000000..70d5f08fe2c
--- /dev/null
+++ b/toolkit/components/passwordmgr/test/test_input_events_for_identical_values.html
@@ -0,0 +1,56 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test for input events in Login Manager when username/password are filled in already</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/EventUtils.js"></script>
+  <script type="text/javascript" src="pwmgr_common.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body onload="onNewEvent(event)">
+Login Manager test: input events should fire.
+
+<script>
+commonInit();
+SimpleTest.waitForExplicitFinish();
+
+/** Test for Login Manager: form fill when form is already filled, should not get input events. **/
+
+var onloadFired = false;
+
+function onNewEvent(e) {
+  console.error("Got " + e.type + " event.");
+  if (e.type == "load") {
+    onloadFired = true;
+    $_(1, "uname").focus();
+    sendKey("Tab");
+  } else {
+    ok(false, "Got an input event for " + e.target.name + " field, which shouldn't happen.");
+  }
+}
+
+SimpleTest.registerCleanupFunction(function cleanup() {
+  $_(1, "uname").removeAttribute("oninput");
+  $_(1, "pword").removeAttribute("onfocus");
+  $_(1, "pword").removeAttribute("oninput");
+  document.body.removeAttribute("onload");
+});
+</script>
+
+<p id="display"></p>
+
+<div id="content">
+
+  <form id="form1" action="formtest.js">
+    <p>This is form 1.</p>
+    <input  type="text"       name="uname" oninput="onNewEvent(event)" value="testuser">
+    <input  type="password"   name="pword" oninput="onNewEvent(event)" onfocus="setTimeout(function() { SimpleTest.finish() }, 1000);" value="testpass">
+
+    <button type="submit">Submit</button>
+    <button type="reset"> Reset </button>
+  </form>
+
+</div>
+<pre id="test"></pre>
+</body>
+</html>
