diff --git a/calendar/providers/caldav/CalDavCalendar.sys.mjs b/calendar/providers/caldav/CalDavCalendar.sys.mjs
index 63a2d08ce5..f7ef29f07a 100644
--- a/calendar/providers/caldav/CalDavCalendar.sys.mjs
+++ b/calendar/providers/caldav/CalDavCalendar.sys.mjs
@@ -1424,21 +1424,15 @@ CalDavCalendar.prototype = {
         this.onPromptAuthAvailable(callback);
       },
       onPromptAuthAvailable(callback) {
-        self.oauth.connect(
+        self.oauth.connect(true, aRefresh).then(
           () => {
             authSuccessCb();
-            if (callback) {
-              callback.onAuthResult(true);
-            }
+            callback?.onAuthResult(true);
           },
           () => {
             authFailureCb();
-            if (callback) {
-              callback.onAuthResult(false);
-            }
-          },
-          true,
-          aRefresh
+            callback?.onAuthResult(false);
+          }
         );
       },
       onPromptCanceled: authFailureCb,
diff --git a/calendar/providers/caldav/modules/CalDavSession.jsm b/calendar/providers/caldav/modules/CalDavSession.jsm
index ac5a301940..af25546285 100644
--- a/calendar/providers/caldav/modules/CalDavSession.jsm
+++ b/calendar/providers/caldav/modules/CalDavSession.jsm
@@ -114,21 +114,15 @@ class CalDavOAuth extends OAuth2 {
           },
 
           onPromptAuthAvailable(callback) {
-            self.connect(
+            self.connect(aWithUI, aRefresh).then(
               () => {
-                if (callback) {
-                  callback.onAuthResult(true);
-                }
+                callback?.onAuthResult(true);
                 resolve();
               },
-              () => {
-                if (callback) {
-                  callback.onAuthResult(false);
-                }
-                reject();
-              },
-              aWithUI,
-              aRefresh
+              error => {
+                callback?.onAuthResult(false);
+                reject(Cr.NS_ERROR_ABORT);
+              }
             );
           },
           onPromptCanceled: reject,
diff --git a/calendar/test/unit/xpcshell.ini b/calendar/test/unit/xpcshell.ini
index 2d4639d0b6..6e26271a3e 100644
--- a/calendar/test/unit/xpcshell.ini
+++ b/calendar/test/unit/xpcshell.ini
@@ -29,6 +29,7 @@ tags = calendar
 [test_bug668222.js]
 [test_bug759324.js]
 [test_caldav_requests.js]
+tags = oauth
 [test_CalendarFileImporter.js]
 [test_calIteratorUtils.js]
 [test_calmgr.js]
diff --git a/mailnews/addrbook/modules/CardDAVUtils.jsm b/mailnews/addrbook/modules/CardDAVUtils.jsm
index 3212b6628d..7961742788 100644
--- a/mailnews/addrbook/modules/CardDAVUtils.jsm
+++ b/mailnews/addrbook/modules/CardDAVUtils.jsm
@@ -378,15 +378,13 @@ var CardDAVUtils = {
       requestParams.oAuth = {
         QueryInterface: ChromeUtils.generateQI(["msgIOAuth2Module"]),
         connect(withUI, listener) {
-          oAuth.connect(
+          oAuth.connect(withUI, false).then(
             () =>
               listener.onSuccess(
                 // String format based on what OAuth2Module has.
                 btoa(`\x01auth=Bearer ${oAuth.accessToken}`)
               ),
-            () => listener.onFailure(Cr.NS_ERROR_ABORT),
-            withUI,
-            false
+            () => listener.onFailure(Cr.NS_ERROR_ABORT)
           );
         },
       };
diff --git a/mailnews/base/src/OAuth2.jsm b/mailnews/base/src/OAuth2.jsm
index b955a256fc..f361a452ff 100644
--- a/mailnews/base/src/OAuth2.jsm
+++ b/mailnews/base/src/OAuth2.jsm
@@ -69,25 +69,37 @@ OAuth2.prototype = {
   refreshToken: null,
   tokenExpires: 0,
 
-  connect(aSuccess, aFailure, aWithUI, aRefresh) {
-    this.connectSuccessCallback = aSuccess;
-    this.connectFailureCallback = aFailure;
-
+  /**
+   * Obtain an access token for this endpoint. If an access token has already
+   * been obtained, it will be reused unless `aRefresh` is true.
+   *
+   * @param {boolean} aWithUI - If UI can be shown to the user for logging in.
+   * @param {boolean} aRefresh - If any existing access token should be
+   *   ignored and a new one obtained.
+   * @returns {Promise} - Resolves when authorisation is complete and an
+   *   access token is available.
+   */
+  connect(aWithUI, aRefresh) {
     if (this.accessToken && !this.tokenExpired && !aRefresh) {
-      aSuccess();
-    } else if (this.refreshToken) {
+      return this._promise;
+    }
+
+    const { promise, resolve, reject } = Promise.withResolvers();
+    this._promise = promise;
+    this._resolve = resolve;
+    this._reject = reject;
+
+    if (this.refreshToken) {
       this.requestAccessToken(this.refreshToken, true);
+    } else if (!aWithUI) {
+      this._reject('{ "error": "auth_noui" }');
+    } else if (gConnecting[this.authorizationEndpoint]) {
+      this._reject("Window already open");
     } else {
-      if (!aWithUI) {
-        aFailure('{ "error": "auth_noui" }');
-        return;
-      }
-      if (gConnecting[this.authorizationEndpoint]) {
-        aFailure("Window already open");
-        return;
-      }
       this.requestAuthorization();
     }
+
+    return this._promise;
   },
 
   /**
@@ -272,7 +284,7 @@ OAuth2.prototype = {
   },
 
   onAuthorizationFailed(aError, aData) {
-    this.connectFailureCallback(aData);
+    this._reject(aData);
   },
 
   /**
@@ -334,11 +346,9 @@ OAuth2.prototype = {
 
           // Typically in production this would be {"error": "invalid_grant"}.
           // That is, the token expired or was revoked (user changed password?).
-          // Reset the tokens we have and call success so that the auth flow
-          // will be re-triggered.
           this.accessToken = null;
           this.refreshToken = null;
-          this.connectSuccessCallback();
+          this._reject(err);
           return;
         }
 
@@ -355,11 +365,11 @@ OAuth2.prototype = {
         } else {
           this.tokenExpires = Number.MAX_VALUE;
         }
-        this.connectSuccessCallback();
+        this._resolve();
       })
       .catch(err => {
         this.log.info(`Connection to authorization server failed: ${err}`);
-        this.connectFailureCallback(err);
+        this._reject(err);
       });
   },
 };
diff --git a/mailnews/base/src/OAuth2Module.sys.mjs b/mailnews/base/src/OAuth2Module.sys.mjs
index 45ae525894..6394664b48 100644
--- a/mailnews/base/src/OAuth2Module.sys.mjs
+++ b/mailnews/base/src/OAuth2Module.sys.mjs
@@ -170,25 +170,19 @@ OAuth2Module.prototype = {
       },
 
       onPromptAuthAvailable: callback => {
-        oauth.connect(
+        oauth.connect(aWithUI, false).then(
           () => {
             aListener.onSuccess(
               btoa(
                 `user=${this._username}\x01auth=Bearer ${oauth.accessToken}\x01\x01`
               )
             );
-            if (callback) {
-              callback.onAuthResult(true);
-            }
+            callback?.onAuthResult(true);
           },
           () => {
             aListener.onFailure(Cr.NS_ERROR_ABORT);
-            if (callback) {
-              callback.onAuthResult(false);
-            }
-          },
-          aWithUI,
-          false
+            callback?.onAuthResult(false);
+          }
         );
       },
       onPromptCanceled() {