diff --git a/toolkit/components/passwordmgr/LoginManager.sys.mjs b/toolkit/components/passwordmgr/LoginManager.sys.mjs
--- a/toolkit/components/passwordmgr/LoginManager.sys.mjs
+++ b/toolkit/components/passwordmgr/LoginManager.sys.mjs
@@ -301,11 +301,10 @@
    */
   initializationPromise: null,
 
   /**
    * Add a new login to login storage.
-   * @deprecated: use `addLoginAsync` instead.
    */
   addLogin(login) {
     this._checkLogin(login);
 
     // Look for an existing entry.
@@ -317,46 +316,14 @@
 
     let matchingLogin = logins.find(l => login.matches(l, true));
     if (matchingLogin) {
       throw lazy.LoginHelper.createLoginAlreadyExistsError(matchingLogin.guid);
     }
-    lazy.log.debug("addLogin is DEPRECATED, please use addLoginAsync instead.");
+    lazy.log.debug("Adding login");
     return this._storage.addLogin(login);
   },
 
-  /**
-   * Add a new login to login storage.
-   */
-  async addLoginAsync(login) {
-    this._checkLogin(login);
-
-    const { origin, formActionOrigin, httpRealm } = login;
-    const existingLogins = this.findLogins(origin, formActionOrigin, httpRealm);
-    const matchingLogin = existingLogins.find(l => login.matches(l, true));
-    if (matchingLogin) {
-      throw lazy.LoginHelper.createLoginAlreadyExistsError(matchingLogin.guid);
-    }
-
-    const crypto = Cc["@mozilla.org/login-manager/crypto/SDR;1"].getService(
-      Ci.nsILoginManagerCrypto
-    );
-    const plaintexts = [login.username, login.password];
-    const [username, password] = await crypto.encryptMany(plaintexts);
-
-    const { username: plaintextUsername, password: plaintextPassword } = login;
-    login.username = username;
-    login.password = password;
-
-    lazy.log.debug("Adding login");
-    return this._storage.addLogin(
-      login,
-      true,
-      plaintextUsername,
-      plaintextPassword
-    );
-  },
-
   async addLogins(logins) {
     if (logins.length === 0) {
       return logins;
     }
 
diff --git a/toolkit/components/passwordmgr/nsILoginManager.idl b/toolkit/components/passwordmgr/nsILoginManager.idl
--- a/toolkit/components/passwordmgr/nsILoginManager.idl
+++ b/toolkit/components/passwordmgr/nsILoginManager.idl
@@ -43,23 +43,10 @@
    * be used instead.
    */
   nsILoginInfo addLogin(in nsILoginInfo aLogin);
 
   /**
-   * Like addLogin, but asynchronous.
-   *
-   * @param aLogin
-   *        The login to be added.
-   * @return A promise which resolves with a cloned login with the guid set.
-   *
-   * Default values for each login's nsILoginMetaInfo properties will be
-   * created. However, if the caller specifies non-default values, they will
-   * be used instead.
-   */
-  Promise addLoginAsync(in nsILoginInfo aLogin);
-
-  /**
    * Like addLogin, but asynchronous and for many logins.
    *
    * @param aLogins
    *        A JS Array of nsILoginInfos to add.
    * @return A promise which resolves with a JS Array of cloned logins with