diff --git a/browser/app/profile/firefox.js b/browser/app/profile/firefox.js
index 95c1befbc58a..7eec89789cc9 100644
--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -1935,6 +1935,7 @@ pref("signon.management.page.breachAlertUrl",
      "https://monitor.firefox.com/breach-details/");
 pref("signon.management.page.showPasswordSyncNotification", true);
 pref("signon.passwordEditCapture.enabled", true);
+pref("signon.relatedRealms.enabled", false);
 pref("signon.showAutoCompleteFooter", true);
 pref("signon.showAutoCompleteImport", "import");
 pref("signon.suggestImportCount", 3);
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index c9122583ce9e..c984a8b0037c 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -3675,6 +3675,7 @@ pref("signon.userInputRequiredToCapture.enabled", true);
 pref("signon.debug",                        false);
 pref("signon.recipes.path", "resource://app/defaults/settings/main/password-recipes.json");
 pref("signon.recipes.remoteRecipesEnabled", true);
+pref("signon.relatedRealms.enabled", false);
 
 pref("signon.schemeUpgrades",                     true);
 pref("signon.includeOtherSubdomainsInLookup",     true);
diff --git a/toolkit/components/passwordmgr/LoginHelper.jsm b/toolkit/components/passwordmgr/LoginHelper.jsm
index 452b681f8f3f..fc1689be2228 100644
--- a/toolkit/components/passwordmgr/LoginHelper.jsm
+++ b/toolkit/components/passwordmgr/LoginHelper.jsm
@@ -371,6 +371,8 @@ this.LoginHelper = {
   privateBrowsingCaptureEnabled: null,
   remoteRecipesEnabled: null,
   remoteRecipesCollection: "password-recipes",
+  relatedRealmsEnabled: null,
+  relatedRealmsCollection: "websites-with-shared-credential-backends",
   schemeUpgrades: null,
   showAutoCompleteFooter: null,
   showAutoCompleteImport: null,
@@ -470,6 +472,9 @@ this.LoginHelper = {
     this.remoteRecipesEnabled = Services.prefs.getBoolPref(
       "signon.recipes.remoteRecipesEnabled"
     );
+    this.relatedRealmsEnabled = Services.prefs.getBoolPref(
+      "signon.relatedRealms.enabled"
+    );
   },
 
   createLogger(aLogPrefix) {
@@ -682,6 +687,8 @@ this.LoginHelper = {
       schemeUpgrades: false,
       acceptWildcardMatch: false,
       acceptDifferentSubdomains: false,
+      acceptRelatedRealms: false,
+      relatedRealms: [],
     }
   ) {
     if (aLoginOrigin == aSearchOrigin) {
@@ -718,6 +725,18 @@ this.LoginHelper = {
         ) {
           return true;
         }
+        if (
+          aOptions.acceptRelatedRealms &&
+          aOptions.relatedRealms.length &&
+          (loginURI.scheme == searchURI.scheme ||
+            (aOptions.schemeUpgrades && schemeMatches))
+        ) {
+          for (let relatedOrigin of aOptions.relatedRealms) {
+            if (Services.eTLD.hasRootDomain(loginURI.host, relatedOrigin)) {
+              return true;
+            }
+          }
+        }
       }
 
       if (
diff --git a/toolkit/components/passwordmgr/LoginManager.jsm b/toolkit/components/passwordmgr/LoginManager.jsm
index 3e75546a7a2b..f65bd26c7e9a 100644
--- a/toolkit/components/passwordmgr/LoginManager.jsm
+++ b/toolkit/components/passwordmgr/LoginManager.jsm
@@ -322,7 +322,6 @@ LoginManager.prototype = {
     if (matchingLogin) {
       throw LoginHelper.createLoginAlreadyExistsError(matchingLogin.guid);
     }
-
     log.debug("Adding login");
     return this._storage.addLogin(login);
   },
diff --git a/toolkit/components/passwordmgr/LoginManagerParent.jsm b/toolkit/components/passwordmgr/LoginManagerParent.jsm
index 6974cc8ea4f9..a660e421e95d 100644
--- a/toolkit/components/passwordmgr/LoginManagerParent.jsm
+++ b/toolkit/components/passwordmgr/LoginManagerParent.jsm
@@ -22,6 +22,13 @@ XPCOMUtils.defineLazyGetter(this, "autocompleteFeature", () => {
   return new ExperimentFeature("password-autocomplete");
 });
 
+XPCOMUtils.defineLazyGetter(this, "LoginRelatedRealmsParent", () => {
+  const { LoginRelatedRealmsParent } = ChromeUtils.import(
+    "resource://gre/modules/LoginRelatedRealms.jsm"
+  );
+  return new LoginRelatedRealmsParent();
+});
+
 XPCOMUtils.defineLazyGlobalGetters(this, ["URL"]);
 
 XPCOMUtils.defineLazyModuleGetters(this, {
@@ -186,6 +193,7 @@ class LoginManagerParent extends JSWindowActorParent {
    * @param {origin?} options.httpRealm To match on. Omit this argument to match all realms.
    * @param {boolean} options.acceptDifferentSubdomains Include results for eTLD+1 matches
    * @param {boolean} options.ignoreActionAndRealm Include all form and HTTP auth logins for the site
+   * @param {string[]} options.relatedRealms Related realms to match against when searching
    */
   static async searchAndDedupeLogins(
     formOrigin,
@@ -194,6 +202,7 @@ class LoginManagerParent extends JSWindowActorParent {
       formActionOrigin,
       httpRealm,
       ignoreActionAndRealm,
+      relatedRealms,
     } = {}
   ) {
     let logins;
@@ -209,6 +218,10 @@ class LoginManagerParent extends JSWindowActorParent {
         matchData.httpRealm = httpRealm;
       }
     }
+    if (LoginHelper.relatedRealmsEnabled) {
+      matchData.acceptRelatedRealms = LoginHelper.relatedRealmsEnabled;
+      matchData.relatedRealms = relatedRealms;
+    }
     try {
       logins = await Services.logins.searchLoginsAsync(matchData);
     } catch (e) {
@@ -535,11 +548,22 @@ class LoginManagerParent extends JSWindowActorParent {
         origin: formOrigin,
       });
     } else {
+      let relatedRealmsOrigins = [];
+      if (LoginHelper.relatedRealmsEnabled) {
+        relatedRealmsOrigins = await LoginRelatedRealmsParent.findRelatedRealms(
+          formOrigin
+        );
+      }
       logins = await LoginManagerParent.searchAndDedupeLogins(formOrigin, {
         formActionOrigin: actionOrigin,
         ignoreActionAndRealm: true,
         acceptDifferentSubdomains: LoginHelper.includeOtherSubdomainsInLookup,
+        relatedRealms: relatedRealmsOrigins,
       });
+      debug(
+        "Adding related logins on page load",
+        logins.map(l => l.origin)
+      );
     }
 
     log("sendLoginDataToChild:", logins.length, "deduped logins");
@@ -606,12 +630,18 @@ class LoginManagerParent extends JSWindowActorParent {
       logins = LoginHelper.vanillaObjectsToLogins(previousResult.logins);
     } else {
       log("Creating new autocomplete search result.");
-
+      let relatedRealmsOrigins = [];
+      if (LoginHelper.relatedRealmsEnabled) {
+        relatedRealmsOrigins = await LoginRelatedRealmsParent.findRelatedRealms(
+          formOrigin
+        );
+      }
       // Autocomplete results do not need to match actionOrigin or exact origin.
       logins = await LoginManagerParent.searchAndDedupeLogins(formOrigin, {
         formActionOrigin: actionOrigin,
         ignoreActionAndRealm: true,
         acceptDifferentSubdomains: LoginHelper.includeOtherSubdomainsInLookup,
+        relatedRealms: relatedRealmsOrigins,
       });
     }
 
diff --git a/toolkit/components/passwordmgr/LoginRelatedRealms.jsm b/toolkit/components/passwordmgr/LoginRelatedRealms.jsm
new file mode 100644
index 000000000000..76333e4763fe
--- /dev/null
+++ b/toolkit/components/passwordmgr/LoginRelatedRealms.jsm
@@ -0,0 +1,118 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
+
+XPCOMUtils.defineLazyModuleGetters(this, {
+  LoginHelper: "resource://gre/modules/LoginHelper.jsm",
+  RemoteSettings: "resource://services-settings/remote-settings.js",
+});
+
+XPCOMUtils.defineLazyGetter(this, "log", () => {
+  let logger = LoginHelper.createLogger("LoginRelatedRealms");
+  return logger;
+});
+
+const EXPORTED_SYMBOLS = ["LoginRelatedRealmsParent"];
+
+class LoginRelatedRealmsParent extends JSWindowActorParent {
+  /**
+   * @type RemoteSettingsClient
+   *
+   * @memberof LoginRelatedRealmsParent
+   */
+  _sharedCredentialsClient = null;
+  /**
+   * @type string[][]
+   *
+   * @memberof LoginRelatedRealmsParent
+   */
+  _relatedDomainsList = [[]];
+
+  /**
+   * Handles the Remote Settings sync event
+   *
+   * @param {Object} aEvent
+   * @param {Array} aEvent.current Records that are currently in the collection after the sync event
+   * @param {Array} aEvent.created Records that were created
+   * @param {Array} aEvent.updated Records that were updated
+   * @param {Array} aEvent.deleted Records that were deleted
+   * @memberof LoginRelatedRealmsParent
+   */
+  onRemoteSettingsSync(aEvent) {
+    let {
+      data: { current },
+    } = aEvent;
+    this._relatedDomainsList = current;
+  }
+
+  async getSharedCredentialsCollection() {
+    if (!this._sharedCredentialsClient) {
+      this._sharedCredentialsClient = RemoteSettings(
+        LoginHelper.relatedRealmsCollection
+      );
+      this._sharedCredentialsClient.on("sync", event =>
+        this.onRemoteSettingsSync(event)
+      );
+      this._relatedDomainsList = await this._sharedCredentialsClient.get();
+      log.debug("Initialized related realms", this._relatedDomainsList);
+    }
+    log.debug("this._relatedDomainsList", this._relatedDomainsList);
+    return this._relatedDomainsList;
+  }
+
+  /**
+   * Determine if there are any related realms of this `formOrigin` using the related realms collection
+   * @param {string} formOrigin A form origin
+   * @return {string[]} filteredRealms An array of domains related to the `formOrigin`
+   * @async
+   * @memberof LoginRelatedRealmsParent
+   */
+  async findRelatedRealms(formOrigin) {
+    try {
+      let formOriginURI = Services.io.newURI(formOrigin);
+      let originDomain = formOriginURI.host;
+      let [
+        { relatedRealms } = {},
+      ] = await this.getSharedCredentialsCollection();
+      if (!relatedRealms) {
+        return [];
+      }
+      let filterOriginIndex;
+      let shouldInclude = false;
+      let filteredRealms = relatedRealms.filter(_realms => {
+        for (let relatedOrigin of _realms) {
+          // We can't have an origin that matches multiple entries in our related realms collection
+          // so we exit the loop early
+          if (shouldInclude) {
+            return false;
+          }
+          if (Services.eTLD.hasRootDomain(originDomain, relatedOrigin)) {
+            shouldInclude = true;
+            break;
+          }
+        }
+        return shouldInclude;
+      });
+      // * Filtered realms is a nested array due to its structure in Remote Settings
+      filteredRealms = filteredRealms.flat();
+
+      filterOriginIndex = filteredRealms.indexOf(originDomain);
+      // Removing the current formOrigin match if it exists in the related realms
+      // so that we don't return duplicates when we search for logins
+      if (filterOriginIndex !== -1) {
+        filteredRealms.splice(filterOriginIndex, 1);
+      }
+      return filteredRealms;
+    } catch (e) {
+      log.error(e);
+      return [];
+    }
+  }
+}
diff --git a/toolkit/components/passwordmgr/moz.build b/toolkit/components/passwordmgr/moz.build
index 1ea21f43cbec..3409a4020ea2 100644
--- a/toolkit/components/passwordmgr/moz.build
+++ b/toolkit/components/passwordmgr/moz.build
@@ -41,6 +41,7 @@ EXTRA_JS_MODULES += [
     "LoginManagerParent.jsm",
     "LoginManagerPrompter.jsm",
     "LoginRecipes.jsm",
+    "LoginRelatedRealms.jsm",
     "NewPasswordModel.jsm",
     "OSCrypto.jsm",
     "PasswordGenerator.jsm",
diff --git a/toolkit/components/passwordmgr/storage-json.js b/toolkit/components/passwordmgr/storage-json.js
index b547f82b14bc..f2bf720e0707 100644
--- a/toolkit/components/passwordmgr/storage-json.js
+++ b/toolkit/components/passwordmgr/storage-json.js
@@ -475,7 +475,9 @@ class LoginManagerStorage_json {
           // Some property names aren't field names but are special options to
           // affect the search.
           case "acceptDifferentSubdomains":
-          case "schemeUpgrades": {
+          case "schemeUpgrades":
+          case "acceptRelatedRealms":
+          case "relatedRealms": {
             options[prop.name] = prop.value;
             break;
           }
@@ -508,6 +510,8 @@ class LoginManagerStorage_json {
     aOptions = {
       schemeUpgrades: false,
       acceptDifferentSubdomains: false,
+      acceptRelatedRealms: false,
+      relatedRealms: [],
     },
     candidateLogins = this._store.data.logins
   ) {
diff --git a/toolkit/components/passwordmgr/test/LoginTestUtils.jsm b/toolkit/components/passwordmgr/test/LoginTestUtils.jsm
index 93329dff431b..bbb6d0da9ebb 100644
--- a/toolkit/components/passwordmgr/test/LoginTestUtils.jsm
+++ b/toolkit/components/passwordmgr/test/LoginTestUtils.jsm
@@ -9,6 +9,14 @@
 
 const EXPORTED_SYMBOLS = ["LoginTestUtils"];
 
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+
+XPCOMUtils.defineLazyModuleGetters(this, {
+  RemoteSettings: "resource://services-settings/remote-settings.js",
+});
+
 const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 let { Assert: AssertCls } = ChromeUtils.import(
@@ -616,3 +624,27 @@ LoginTestUtils.file = {
     return tmpFile;
   },
 };
+
+LoginTestUtils.remoteSettings = {
+  relatedRealmsCollection: "websites-with-shared-credential-backends",
+  async setupWebsitesWithSharedCredentials(
+    relatedRealms = [["other-example.com", "example.com", "example.co.uk"]]
+  ) {
+    let db = await RemoteSettings(this.relatedRealmsCollection).db;
+    await db.clear();
+    await db.create({
+      id: "some-fake-ID-abc",
+      relatedRealms,
+    });
+    await db.importChanges({}, 1234567);
+  },
+  async cleanWebsitesWithSharedCredentials() {
+    let db = await RemoteSettings(this.relatedRealmsCollection).db;
+    await db.clear();
+    await db.importChanges({}, 1234);
+  },
+  async updateTimestamp() {
+    let db = await RemoteSettings(this.relatedRealmsCollection).db;
+    await db.importChanges({}, 12345678);
+  },
+};
diff --git a/toolkit/components/passwordmgr/test/browser/head.js b/toolkit/components/passwordmgr/test/browser/head.js
index 0a2316c173e0..b3c504a6aeb6 100644
--- a/toolkit/components/passwordmgr/test/browser/head.js
+++ b/toolkit/components/passwordmgr/test/browser/head.js
@@ -28,6 +28,12 @@ add_task(async function common_initialize() {
       ["toolkit.telemetry.ipcBatchTimeout", 0],
     ],
   });
+  if (LoginHelper.relatedRealmsEnabled) {
+    LoginTestUtils.remoteSettings.setupWebsitesWithSharedCredentials();
+    registerCleanupFunction(function() {
+      LoginTestUtils.remoteSettings.cleanWebsitesWithSharedCredentials();
+    });
+  }
 });
 
 registerCleanupFunction(
diff --git a/toolkit/components/passwordmgr/test/mochitest/mochitest.ini b/toolkit/components/passwordmgr/test/mochitest/mochitest.ini
index 330bd1bc6e2d..079246d77c57 100644
--- a/toolkit/components/passwordmgr/test/mochitest/mochitest.ini
+++ b/toolkit/components/passwordmgr/test/mochitest/mochitest.ini
@@ -7,6 +7,8 @@ prefs =
   signon.testOnlyUserHasInteractedWithDocument=true
   security.insecure_field_warning.contextual.enabled=false
   network.auth.non-web-content-triggered-resources-http-auth-allow=true
+  # signon.relatedRealms.enabled pref needed until Bug 1699698 lands
+  signon.relatedRealms.enabled=true
 
 support-files =
   ../../../prompts/test/chromeScript.js
@@ -38,6 +40,8 @@ skip-if = toolkit == 'android' && !is_fennec # Don't run on GeckoView
 
 # Note: new tests should use scheme = https unless they have a specific reason not to
 
+[test_autocomplete_autofill_related_realms_no_dupes.html]
+scheme = https
 [test_autocomplete_basic_form.html]
 skip-if = toolkit == 'android' || debug && (os == 'linux' || os == 'win') || os == 'linux' && tsan # android:autocomplete. Bug 1541945, Bug 1590928
 scheme = https
@@ -46,6 +50,8 @@ skip-if = toolkit == 'android' || os == 'linux' # android:autocomplete., linux:
 [test_autocomplete_basic_form_formActionOrigin.html]
 skip-if = toolkit == 'android' # android:autocomplete.
 scheme = https
+[test_autocomplete_basic_form_related_realms.html]
+scheme = https
 [test_autocomplete_basic_form_subdomain.html]
 skip-if = toolkit == 'android' # android:autocomplete.
 scheme = https
diff --git a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
index 22a7b65583ab..75a47ef72fec 100644
--- a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
+++ b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common.js
@@ -667,6 +667,23 @@ function resetRecipes() {
   });
 }
 
+function resetWebsitesWithSharedCredential() {
+  info("Resetting the 'websites-with-shared-credential-backend' collection");
+  return new Promise(resolve => {
+    PWMGR_COMMON_PARENT.addMessageListener(
+      "resetWebsitesWithSharedCredential",
+      function reset() {
+        PWMGR_COMMON_PARENT.removeMessageListener(
+          "resetWebsitesWithSharedCredential",
+          reset
+        );
+        resolve();
+      }
+    );
+    PWMGR_COMMON_PARENT.sendAsyncMessage("resetWebsitesWithSharedCredential");
+  });
+}
+
 function promiseStorageChanged(expectedChangeTypes) {
   return new Promise((resolve, reject) => {
     function onStorageChanged({ topic, data }) {
diff --git a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
index a983950d03e6..93666911034c 100644
--- a/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
+++ b/toolkit/components/passwordmgr/test/mochitest/pwmgr_common_parent.js
@@ -21,6 +21,12 @@ var { LoginManagerParent } = ChromeUtils.import(
 const { LoginTestUtils } = ChromeUtils.import(
   "resource://testing-common/LoginTestUtils.jsm"
 );
+if (LoginHelper.relatedRealmsEnabled) {
+  let rsPromise = LoginTestUtils.remoteSettings.setupWebsitesWithSharedCredentials();
+  async () => {
+    await rsPromise;
+  };
+}
 var { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 /**
@@ -127,6 +133,7 @@ addMessageListener("cleanup", () => {
   Services.obs.removeObserver(onStorageChanged, "passwordmgr-storage-changed");
   Services.obs.removeObserver(onPrompt, "passwordmgr-prompt-change");
   Services.obs.removeObserver(onPrompt, "passwordmgr-prompt-save");
+  Services.logins.removeAllUserFacingLogins();
 });
 
 // Begin message listeners
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_DOMInputPasswordAdded_fired_between_DOMContentLoaded_and_load_events.html b/toolkit/components/passwordmgr/test/mochitest/test_DOMInputPasswordAdded_fired_between_DOMContentLoaded_and_load_events.html
index a49aaaf78861..31d9683a483a 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_DOMInputPasswordAdded_fired_between_DOMContentLoaded_and_load_events.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_DOMInputPasswordAdded_fired_between_DOMContentLoaded_and_load_events.html
@@ -25,7 +25,7 @@ let FILE_PATH = "/tests/toolkit/components/passwordmgr/test/mochitest/slow_image
 runInParent(function removeAll() {
   let {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
   Services.logins.removeAllUserFacingLogins();
-})
+});
 
 let readyPromise = registerRunTests();
 
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_autofill_related_realms_no_dupes.html b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_autofill_related_realms_no_dupes.html
new file mode 100644
index 000000000000..b32ae69e34b1
--- /dev/null
+++ b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_autofill_related_realms_no_dupes.html
@@ -0,0 +1,101 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test no duplicate logins using autofill/autocomplete with related realms</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="/tests/SimpleTest/EventUtils.js"></script>
+  <script type="text/javascript" src="../../../satchel/test/satchel_common.js"></script>
+  <script type="text/javascript" src="pwmgr_common.js"></script>
+  <link rel="stylesheet" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+Login Manager test: no duplicate logins when using autofill and autocomplete with related realms
+<script>
+  addLoginsInParent(
+    // Simple related domain relationship where example.com and other-example.com are in the related domains list
+    ["https://other-example.com", "https://other-example.com", null, "relatedUser1", "relatedPass1", "uname", "pword"],
+
+    // Example.com and example.co.uk are related, so sub.example.co.uk should appear on example.com's autocomplete dropdown
+    // The intent is to cover the ebay.com/ebay.co.uk and all other country TLD cases
+    // where the sign in page is actually signin.ebay.com/signin.ebay.co.uk but credentials could have manually been entered
+    // for ebay.com/ebay.co.uk or automatically stored as signin.ebay.com/sigin.ebay.co.uk
+    ["https://sub.example.co.uk", "https://sub.example.co.uk", null, "subUser1", "subPass1", "uname", "pword"],
+
+    // Ensures there are no duplicates for the exact domain that the user is on
+    ["https://example.com", "https://example.com", null, "exactUser1", "exactPass1", "uname", "pword"],
+    ["https://www.example.com", "https://www.example.com", null, "exactWWWUser1", "exactWWWPass1", "uname", "pword"],
+    );
+</script>
+<p id="display"></p>
+<div id="content">
+  <form id="form1" action="https://www.example.com" onsubmit="return false;">
+    <input type="text" name="uname">
+    <input type="password" name="pword">
+    <button type="submit">Submit</button>
+  </form>
+
+</div>
+
+<pre id="test">
+<script class="testbody" type="text/javascript">
+
+/** Test for Login Manager: related realms autofill/autocomplete. **/
+
+let uname = $_(1, "uname");
+let pword = $_(1, "pword");
+
+function restoreForm() {
+  uname.value = "";
+  pword.value = "";
+  uname.focus();
+}
+
+function sendFakeAutocompleteEvent(element) {
+  var acEvent = document.createEvent("HTMLEvents");
+  acEvent.initEvent("DOMAutoComplete", true, false);
+  element.dispatchEvent(acEvent);
+}
+
+function spinEventLoop() {
+  return Promise.resolve();
+}
+
+async function promiseACPopupClosed() {
+  return SimpleTest.promiseWaitForCondition(async () => {
+    let popupState = await getPopupState();
+    return !popupState.open;
+  }, "Wait for AC popup to be closed");
+}
+
+add_task(async function setup() {
+  listenForUnexpectedPopupShown();
+});
+
+add_task(async function test_no_duplicates_autocomplete_autofill() {
+  await promiseFormsProcessedInSameProcess();
+  await SimpleTest.promiseFocus(window);
+  let shownPromise = promiseACShown();
+  checkLoginForm(uname, "exactUser1", pword, "exactPass1")
+  restoreForm();
+  let results = await shownPromise;
+
+  let popupState = await getPopupState();
+  is(popupState.selectedIndex, -1, "Check no entires are selected upon opening");
+  let expectedMenuItems = ["exactUser1", "exactWWWUser1", "relatedUser1", "subUser1"];
+  checkAutoCompleteResults(results, expectedMenuItems, window.location.host, "Check all menuitems are displayed correctly");
+
+  let acEvents = await getTelemetryEvents({ process: "parent", filterProps: TelemetryFilterPropsAC, clear: true });
+  is(acEvents.length, 1, "One autocomplete event");
+  checkACTelemetryEvent(acEvents[0], uname, {
+    "hadPrevious": "0",
+    "login": expectedMenuItems.length + "",
+    "loginsFooter": "1"
+  });
+  restoreForm();
+  synthesizeKey("KEY_Escape");
+});
+</script>
+</pre>
+</body>
+</html>
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_basic_form_related_realms.html b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_basic_form_related_realms.html
new file mode 100644
index 000000000000..3951e9e19eed
--- /dev/null
+++ b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_basic_form_related_realms.html
@@ -0,0 +1,131 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test login autocomplete with related realms</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="/tests/SimpleTest/EventUtils.js"></script>
+  <script type="text/javascript" src="../../../satchel/test/satchel_common.js"></script>
+  <script type="text/javascript" src="pwmgr_common.js"></script>
+  <link rel="stylesheet" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+Login Manager test: related realms autocomplete
+
+<script>
+  addLoginsInParent(
+    // Simple related domain relationship where example.com and other-example.com are in the related domains list
+    ["https://other-example.com", "https://other-example.com", null, "relatedUser1", "relatedPass1", "uname", "pword"],
+
+    // Example.com and example.co.uk are related, so sub.example.co.uk should appear on example.com's autocomplete dropdown
+    // The intent is to cover the ebay.com/ebay.co.uk and all other country TLD cases
+    // where the sign in page is actually signin.ebay.com/signin.ebay.co.uk but credentials could have manually been entered
+    // for ebay.com/ebay.co.uk or automatically stored as signin.ebay.com/sigin.ebay.co.uk
+    ["https://sub.example.co.uk", "https://sub.example.co.uk", null, "subUser1", "subPass1", "uname", "pword"],
+    );
+</script>
+<p id="display"></p>
+<div id="content">
+  <form id="form1" onsubmit="return false;">
+    <input type="text" name="uname">
+    <input type="password" name="pword">
+    <button type="submit">Submit</button>
+  </form>
+
+</div>
+
+<pre id="test">
+<script class="testbody" type="text/javascript">
+
+/** Test for Login Manager: related realms autocomplete. **/
+
+let uname = $_(1, "uname");
+let pword = $_(1, "pword");
+
+function restoreForm() {
+  uname.value = "";
+  pword.value = "";
+  uname.focus();
+}
+
+function sendFakeAutocompleteEvent(element) {
+  var acEvent = document.createEvent("HTMLEvents");
+  acEvent.initEvent("DOMAutoComplete", true, false);
+  element.dispatchEvent(acEvent);
+}
+
+function spinEventLoop() {
+  return Promise.resolve();
+}
+
+async function promiseACPopupClosed() {
+  return SimpleTest.promiseWaitForCondition(async () => {
+    let popupState = await getPopupState();
+    return !popupState.open;
+  }, "Wait for AC popup to be closed");
+}
+
+add_task(async function setup() {
+  listenForUnexpectedPopupShown();
+});
+
+add_task(async function test_form1_initial_empty() {
+  await SimpleTest.promiseFocus(window);
+
+  // Make sure initial form is empty.
+  checkLoginForm(uname, "", pword, "");
+  let popupState = await getPopupState();
+  is(popupState.open, false, "Check popup is initially closed");
+});
+
+add_task(async function test_form_related_domain_menuitems() {
+  await SimpleTest.promiseFocus(window);
+  let shownPromise = promiseACShown();
+  // Trigger autocomplete popup
+  restoreForm();
+  let results = await shownPromise;
+  let popupState = await getPopupState();
+  is(popupState.selectedIndex, -1, "Check no entires are selected upon opening");
+
+  let expectedMenuItems = ["relatedUser1", "subUser1"];
+  checkAutoCompleteResults(results, expectedMenuItems, window.location.host, "Check all menuitems are displayed correctly");
+
+  let acEvents = await getTelemetryEvents({ process: "parent", filterProps: TelemetryFilterPropsAC, clear: true });
+  is(acEvents.length, 1, "One autocomplete event");
+  checkACTelemetryEvent(acEvents[0], uname, {
+    "hadPrevious": "0",
+    "login": expectedMenuItems.length + "",
+    "loginsFooter": "1"
+  });
+  checkLoginForm(uname, "", pword, ""); // value shouldn't update just by opening
+
+  synthesizeKey("KEY_ArrowDown"); // first item
+  checkLoginForm(uname, "", pword, ""); // value shouldn't update just by selecting
+
+  synthesizeKey("KEY_Enter");
+  await promiseFormsProcessedInSameProcess();
+  is(pword.value, "relatedPass1", "password should match the login that was selected");
+  checkLoginForm(uname, "relatedUser1", pword, "relatedPass1");
+
+  restoreForm();
+
+  shownPromise = promiseACShown();
+  synthesizeKey("KEY_ArrowDown"); // open
+  await shownPromise;
+
+  synthesizeKey("KEY_ArrowDown"); // first item
+  synthesizeKey("KEY_ArrowDown"); // second item
+  checkLoginForm(uname, "", pword, ""); // value shouldn't update just by selecting
+
+  synthesizeKey("KEY_Enter");
+  await promiseFormsProcessedInSameProcess();
+  is(pword.value, "subPass1", "password should match the login that was selected");
+  checkLoginForm(uname, "subUser1", pword, "subPass1");
+
+  restoreForm();
+});
+
+</script>
+</pre>
+</body>
+</html>
diff --git a/toolkit/components/passwordmgr/test/mochitest/test_username_focus.html b/toolkit/components/passwordmgr/test/mochitest/test_username_focus.html
index 4bbfa2f1d4b2..0008087dc074 100644
--- a/toolkit/components/passwordmgr/test/mochitest/test_username_focus.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_username_focus.html
@@ -240,10 +240,6 @@ add_task(async function test_multiple_prefilled_focused_dynamic() {
   usernameField.focus();
   await shownPromise;
 });
-
-add_task(async function cleanup() {
-  removeFocus();
-});
 </script>
 </pre>
 </body>
diff --git a/toolkit/components/passwordmgr/test/unit/head.js b/toolkit/components/passwordmgr/test/unit/head.js
index e7c9205257d3..57682221b114 100644
--- a/toolkit/components/passwordmgr/test/unit/head.js
+++ b/toolkit/components/passwordmgr/test/unit/head.js
@@ -52,7 +52,7 @@ const newPropertyBag = LoginHelper.newPropertyBag;
 
 const NEW_PASSWORD_HEURISTIC_ENABLED_PREF =
   "signon.generation.confidenceThreshold";
-
+const RELATED_REALMS_ENABLED_PREF = "signon.relatedRealms.enabled";
 /**
  * All the tests are implemented with add_task, this starts them automatically.
  */
@@ -91,6 +91,12 @@ add_task(async function test_common_initialize() {
 
   // Ensure that the service and the storage module are initialized.
   await Services.logins.initializationPromise;
+  Services.prefs.setBoolPref(RELATED_REALMS_ENABLED_PREF, true);
+  if (LoginHelper.relatedRealmsEnabled) {
+    // Ensure that there is a mocked Remote Settings database for the
+    // "websites-with-shared-credential-backends" collection
+    await LoginTestUtils.remoteSettings.setupWebsitesWithSharedCredentials();
+  }
 });
 
 add_task(async function test_common_prefs() {
diff --git a/toolkit/components/passwordmgr/test/unit/test_findRelatedRealms.js b/toolkit/components/passwordmgr/test/unit/test_findRelatedRealms.js
new file mode 100644
index 000000000000..1f9bbc718c71
--- /dev/null
+++ b/toolkit/components/passwordmgr/test/unit/test_findRelatedRealms.js
@@ -0,0 +1,157 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { LoginRelatedRealmsParent } = ChromeUtils.import(
+  "resource://gre/modules/LoginRelatedRealms.jsm"
+);
+const { RemoteSettings } = ChromeUtils.import(
+  "resource://services-settings/remote-settings.js",
+  {}
+);
+
+const REMOTE_SETTINGS_COLLECTION = "websites-with-shared-credential-backends";
+
+add_task(async function test_related_domain_matching() {
+  const client = RemoteSettings(REMOTE_SETTINGS_COLLECTION);
+  const records = await client.get();
+  console.log(records);
+
+  // Assumes that the test collection is a 2D array with one subarray
+  let relatedRealms = records[0].relatedRealms;
+  relatedRealms = relatedRealms.flat();
+  ok(relatedRealms);
+
+  let LRR = new LoginRelatedRealmsParent();
+
+  // We should not return unrelated realms
+  let result = await LRR.findRelatedRealms("https://not-example.com");
+  equal(result.length, 0, "Check that there were no related realms found");
+
+  // We should not return unrelated realms given an unrelated subdomain
+  result = await LRR.findRelatedRealms("https://sub.not-example.com");
+  equal(result.length, 0, "Check that there were no related realms found");
+  // We should return the related realms collection
+  result = await LRR.findRelatedRealms("https://sub.example.com");
+  equal(
+    result.length,
+    relatedRealms.length,
+    "Ensure that three related realms were found"
+  );
+
+  // We should return the related realms collection minus the base domain that we searched with
+  result = await LRR.findRelatedRealms("https://example.co.uk");
+  equal(
+    result.length,
+    relatedRealms.length - 1,
+    "Ensure that two related realms were found"
+  );
+});
+
+add_task(async function test_newly_synced_collection() {
+  // Initialize LoginRelatedRealmsParent so the sync handler is enabled
+  let LRR = new LoginRelatedRealmsParent();
+  await LRR.getSharedCredentialsCollection();
+
+  const client = RemoteSettings(REMOTE_SETTINGS_COLLECTION);
+  let records = await client.get();
+  const record1 = {
+    id: records[0].id,
+    relatedRealms: records[0].relatedRealms,
+  };
+
+  // Assumes that the test collection is a 2D array with one subarray
+  let originalRelatedRealms = records[0].relatedRealms;
+  originalRelatedRealms = originalRelatedRealms.flat();
+  ok(originalRelatedRealms);
+
+  const updatedRelatedRealms = ["completely-different.com", "example.com"];
+  const record2 = {
+    id: "some-other-ID",
+    relatedRealms: [updatedRelatedRealms],
+  };
+  const payload = {
+    current: [record2],
+    created: [record2],
+    updated: [],
+    deleted: [record1],
+  };
+  await RemoteSettings(REMOTE_SETTINGS_COLLECTION).emit("sync", {
+    data: payload,
+  });
+
+  let [{ id, relatedRealms }] = await LRR.getSharedCredentialsCollection();
+  equal(id, record2.id, "internal collection ID should be updated");
+  equal(
+    relatedRealms,
+    record2.relatedRealms,
+    "internal collection related realms should be updated"
+  );
+
+  // We should return only one result, and that result should be example.com
+  // NOT other-example.com or example.co.uk
+  let result = await LRR.findRelatedRealms("https://completely-different.com");
+  equal(
+    result.length,
+    updatedRelatedRealms.length - 1,
+    "Check that there is only one related realm found"
+  );
+  equal(
+    result[0],
+    "example.com",
+    "Ensure that the updated collection should only match example.com"
+  );
+});
+
+add_task(async function test_no_related_domains() {
+  await LoginTestUtils.remoteSettings.cleanWebsitesWithSharedCredentials();
+
+  const client = RemoteSettings(REMOTE_SETTINGS_COLLECTION);
+  let records = await client.get();
+
+  equal(records.length, 0, "Check that there are no related realms");
+
+  let LRR = new LoginRelatedRealmsParent();
+
+  ok(LRR.findRelatedRealms, "Ensure findRelatedRealms exists");
+
+  let result = await LRR.findRelatedRealms("https://example.com");
+  equal(result.length, 0, "Assert that there were no related realms found");
+});
+
+add_task(async function test_unrelated_subdomains() {
+  await LoginTestUtils.remoteSettings.cleanWebsitesWithSharedCredentials();
+  let testCollection = [
+    ["slpl.bibliocommons.com", "slpl.overdrive.com"],
+    ["springfield.overdrive.com", "coolcat.org"],
+  ];
+  await LoginTestUtils.remoteSettings.setupWebsitesWithSharedCredentials(
+    testCollection
+  );
+
+  let LRR = new LoginRelatedRealmsParent();
+  let result = await LRR.findRelatedRealms("https://evil.overdrive.com");
+  equal(result.length, 0, "Assert that there were no related realms found");
+
+  result = await LRR.findRelatedRealms("https://abc.slpl.bibliocommons.com");
+  equal(result.length, 2, "Assert that two related realms were found");
+  equal(result[0], testCollection[0][0]);
+  equal(result[1], testCollection[0][1]);
+
+  result = await LRR.findRelatedRealms("https://slpl.overdrive.com");
+  console.log("what is result: " + result);
+  equal(result.length, 1, "Assert that one related realm was found");
+  for (let item of result) {
+    notEqual(
+      item,
+      "coolcat.org",
+      "coolcat.org is not related to slpl.overdrive.com"
+    );
+    notEqual(
+      item,
+      "springfield.overdrive.com",
+      "springfield.overdrive.com is not related to slpl.overdrive.com"
+    );
+  }
+});
diff --git a/toolkit/components/passwordmgr/test/unit/test_remote_recipes.js b/toolkit/components/passwordmgr/test/unit/test_remote_recipes.js
index 4197865f09b2..07ef3055f6bf 100644
--- a/toolkit/components/passwordmgr/test/unit/test_remote_recipes.js
+++ b/toolkit/components/passwordmgr/test/unit/test_remote_recipes.js
@@ -3,6 +3,7 @@
 
 /**
  * Tests retrieving remote LoginRecipes in the parent process.
+ * See https://firefox-source-docs.mozilla.org/services/settings/#unit-tests for explanation of db.importChanges({}, 42);
  */
 
 "use strict";
@@ -16,13 +17,14 @@ const REMOTE_SETTINGS_COLLECTION = "password-recipes";
 
 add_task(async function test_init_remote_recipe() {
   const db = await RemoteSettings(REMOTE_SETTINGS_COLLECTION).db;
+  await db.clear();
   const record1 = {
     id: "some-fake-ID",
     hosts: ["www.testDomain.com"],
     description: "Some description here",
     usernameSelector: "#username",
   };
-  await db.create(record1);
+  await db.importChanges({}, 42, [record1], { clear: true });
   let parent = new LoginRecipesParent({ defaults: true });
 
   let recipesParent = await parent.initializationPromise;
@@ -54,6 +56,7 @@ add_task(async function test_init_remote_recipe() {
     "Initially 1 recipe based on our test record"
   );
   await db.clear();
+  await db.importChanges({}, 42);
 });
 
 add_task(async function test_add_recipe_sync() {
@@ -64,7 +67,7 @@ add_task(async function test_add_recipe_sync() {
     description: "Some description here",
     usernameSelector: "#username",
   };
-  await db.create(record1);
+  await db.importChanges({}, 42, [record1], { clear: true });
   let parent = new LoginRecipesParent({ defaults: true });
   let recipesParent = await parent.initializationPromise;
 
@@ -89,6 +92,7 @@ add_task(async function test_add_recipe_sync() {
     "New recipe from sync event added successfully"
   );
   await db.clear();
+  await db.importChanges({}, 42);
 });
 
 add_task(async function test_remove_recipe_sync() {
@@ -99,7 +103,7 @@ add_task(async function test_remove_recipe_sync() {
     description: "Some description here",
     usernameSelector: "#username",
   };
-  await db.create(record1);
+  await db.importChanges({}, 42, [record1], { clear: true });
   let parent = new LoginRecipesParent({ defaults: true });
   let recipesParent = await parent.initializationPromise;
 
@@ -129,7 +133,7 @@ add_task(async function test_malformed_recipes_in_db() {
     usernameSelector: "#username",
     fieldThatDoesNotExist: "value",
   };
-  await db.create(malformedRecord);
+  await db.importChanges({}, 42, [malformedRecord], { clear: true });
   let parent = new LoginRecipesParent({ defaults: true });
   try {
     await parent.initializationPromise;
@@ -146,7 +150,7 @@ add_task(async function test_malformed_recipes_in_db() {
     description: "Some description here",
     usernameSelector: "#username",
   };
-  await db.create(missingHostsRecord);
+  await db.importChanges({}, 42, [missingHostsRecord], { clear: true });
   parent = new LoginRecipesParent({ defaults: true });
   try {
     await parent.initializationPromise;
diff --git a/toolkit/components/passwordmgr/test/unit/xpcshell.ini b/toolkit/components/passwordmgr/test/unit/xpcshell.ini
index a1c343fa379f..3fbcfdca6357 100644
--- a/toolkit/components/passwordmgr/test/unit/xpcshell.ini
+++ b/toolkit/components/passwordmgr/test/unit/xpcshell.ini
@@ -18,6 +18,7 @@ run-if = buildapp == "browser"
 [test_disabled_hosts.js]
 [test_displayOrigin.js]
 [test_doLoginsMatch.js]
+[test_findRelatedRealms.js]
 [test_getFormFields.js]
 [test_getPasswordFields.js]
 [test_getPasswordOrigin.js]