diff --git a/toolkit/components/passwordmgr/LoginManagerContent.jsm b/toolkit/components/passwordmgr/LoginManagerContent.jsm
index cc976137b51e..37ab8a2c6308 100644
--- a/toolkit/components/passwordmgr/LoginManagerContent.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerContent.jsm
@@ -1530,12 +1530,13 @@ this.LoginManagerContent = {
 
   _maybeStopTreatingAsGeneratedPasswordField(event) {
     let passwordField = event.target;
+    let { value } = passwordField;
 
-    // If the field isn't empty then keep treating it as a generated password field.
-    if (passwordField.value) {
-      return;
+    // If the field is now empty or the inserted text replaced the whole value
+    // then stop treating it as a generated password field.
+    if (!value || (event.data && event.data == value)) {
+      this._stopTreatingAsGeneratedPasswordField(passwordField);
     }
-    this._stopTreatingAsGeneratedPasswordField(passwordField);
   },
 
   _stopTreatingAsGeneratedPasswordField(passwordField) {
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_LoginManagerContent_generatedPasswordFilledOrEdited.html b/toolkit/components/passwordmgr/test/mochitest/test_LoginManagerContent_generatedPasswordFilledOrEdited.html
index 4edd23594b44..3d6fd511a3e9 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_LoginManagerContent_generatedPasswordFilledOrEdited.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_LoginManagerContent_generatedPasswordFilledOrEdited.html
@@ -94,6 +94,61 @@ add_task(async function test_fieldsMaskedAfterSavedLoginFill() {
   await promiseFormsProcessed();
 });
 
+add_task(async function test_fieldsMaskedAfterReplacingWholeValue() {
+  let pword = $_(1, "pword");
+  pword.focus();
+
+  SpecialPowers.wrap(pword).setUserInput("generatedpass");
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "Before first fill");
+  LoginManagerContent._generatedPasswordFilledOrEdited(pword);
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, false, "After first fill");
+  synthesizeKey("KEY_Tab", { shiftKey: true }); // blur pw, focus un
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "After blur");
+
+  synthesizeKey("KEY_Tab"); // focus again and replace the whole password value
+  info("Replacing password field value with arbitrary string");
+  sendString("some_other_password");
+  is(pword.value, "some_other_password", "Whole password replaced")
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "Replaced password value");
+
+  synthesizeKey("KEY_Tab"); // blur pw
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "After blur");
+  synthesizeKey("KEY_Tab", { shiftKey: true }); // focus pw again
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "After focus again");
+
+  // Cleanup
+  recreateTree(document.getElementById("form1"));
+  await promiseFormsProcessed();
+});
+
+add_task(async function test_fieldsUnmaskedAfterAddingCharacter() {
+  let pword = $_(1, "pword");
+  pword.focus();
+
+  SpecialPowers.wrap(pword).setUserInput("generatedpass");
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "Before first fill");
+  LoginManagerContent._generatedPasswordFilledOrEdited(pword);
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, false, "After first fill");
+  synthesizeKey("KEY_Tab", { shiftKey: true }); // blur pw, focus un
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "After blur");
+
+  synthesizeKey("KEY_Tab"); // focus again
+  synthesizeKey("KEY_ArrowRight"); // Remove the selection
+  info("Adding a character to the end of the password");
+  sendString("@");
+  is(pword.value, "generatedpass@", "Character was added to the value")
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, false, "Added @");
+
+  synthesizeKey("KEY_Tab"); // blur pw
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, true, "After blur after @");
+  synthesizeKey("KEY_Tab", { shiftKey: true }); // focus pw again
+  LOGIN_FIELD_UTILS.checkPasswordMasked(pword, false, "After focus after @");
+
+  // Cleanup
+  recreateTree(document.getElementById("form1"));
+  await promiseFormsProcessed();
+});
+
 add_task(async function test_typeNotPassword() {
   let pword = $_(1, "pword");
   pword.focus();