--- a/toolkit/locales/en-US/chrome/mozapps/update/updates.properties
+++ b/toolkit/locales/en-US/chrome/mozapps/update/updates.properties
@@ -64,16 +64,17 @@ applyingUpdate=Applying update…
 
 updatesfound_minor.title=Update Available
 updatesfound_major.title=New Version Available
 
 installSuccess=The Update was successfully installed
 installPending=Install Pending
 patchApplyFailure=The Update could not be installed (patch apply failed)
 elevationFailure=You don’t have the permissions necessary to install this update. Please contact your system administrator.
+appInUseFailure=Another instance of the application prevented updating.
 
 # LOCALIZATION NOTE: %S is the amount downloaded so far
 # example: Paused —  879 KB of 2.1 MB
 downloadPausedStatus=Paused —  %S
 
 check_error-200=Update XML file malformed (200)
 check_error-403=Access denied (403)
 check_error-404=Update XML file not found (404)

--- a/toolkit/mozapps/update/common/errors.h
+++ b/toolkit/mozapps/update/common/errors.h
@@ -91,16 +91,19 @@
 #define INVALID_APPLYTO_DIR_STAGED_ERROR 72
 #define LOCK_ERROR_PATCH_FILE 73
 #define INVALID_APPLYTO_DIR_ERROR 74
 #define INVALID_INSTALL_DIR_PATH_ERROR 75
 #define INVALID_WORKING_DIR_PATH_ERROR 76
 #define INVALID_CALLBACK_PATH_ERROR 77
 #define INVALID_CALLBACK_DIR_ERROR 78
 
+#define CALLBACK_ERROR_WAIT_PID 81
+#define CALLBACK_ERROR_SHARING_VIOLATION 82
+
 // Error codes 80 through 99 are reserved for nsUpdateService.js
 
 // The following error codes are only used by updater.exe
 // when a fallback key exists for tests.
 #define FALLBACKKEY_UNKNOWN_ERROR 100
 #define FALLBACKKEY_REGPATH_ERROR 101
 #define FALLBACKKEY_NOKEY_ERROR 102
 #define FALLBACKKEY_SERVICE_NO_STOP_ERROR 103

--- a/toolkit/mozapps/update/nsUpdateService.js
+++ b/toolkit/mozapps/update/nsUpdateService.js
@@ -43,32 +43,35 @@ const PREF_APP_UPDATE_SERVICE_ENABLED   
 const PREF_APP_UPDATE_SERVICE_ERRORS       = "app.update.service.errors";
 const PREF_APP_UPDATE_SERVICE_MAXERRORS    = "app.update.service.maxErrors";
 const PREF_APP_UPDATE_SILENT               = "app.update.silent";
 const PREF_APP_UPDATE_SOCKET_MAXERRORS     = "app.update.socket.maxErrors";
 const PREF_APP_UPDATE_SOCKET_RETRYTIMEOUT  = "app.update.socket.retryTimeout";
 const PREF_APP_UPDATE_STAGING_ENABLED      = "app.update.staging.enabled";
 const PREF_APP_UPDATE_URL                  = "app.update.url";
 const PREF_APP_UPDATE_URL_DETAILS          = "app.update.url.details";
+const PREF_APP_UPDATE_IN_USE_FAIL_TIME     = "app.update.inUseFailure.time";
+const PREF_APP_UPDATE_IN_USE_FAIL_VERSION  = "app.update.inUseFailure.version";
 
 const URI_BRAND_PROPERTIES      = "chrome://branding/locale/brand.properties";
 const URI_UPDATE_HISTORY_DIALOG = "chrome://mozapps/content/update/history.xul";
 const URI_UPDATE_NS             = "http://www.mozilla.org/2005/app-update";
 const URI_UPDATE_PROMPT_DIALOG  = "chrome://mozapps/content/update/updates.xul";
 const URI_UPDATES_PROPERTIES    = "chrome://mozapps/locale/update/updates.properties";
 
 const KEY_UPDROOT         = "UpdRootD";
 const KEY_EXECUTABLE      = "XREExeF";
 
 const DIR_UPDATES         = "updates";
 
 const FILE_ACTIVE_UPDATE_XML = "active-update.xml";
 const FILE_BACKUP_UPDATE_LOG = "backup-update.log";
 const FILE_LAST_UPDATE_LOG   = "last-update.log";
 const FILE_UPDATES_XML       = "updates.xml";
+const FILE_UPDATE_FORCE      = "update.force";
 const FILE_UPDATE_LOG        = "update.log";
 const FILE_UPDATE_MAR        = "update.mar";
 const FILE_UPDATE_STATUS     = "update.status";
 const FILE_UPDATE_TEST       = "update.test";
 const FILE_UPDATE_VERSION    = "update.version";
 
 const STATE_NONE            = "null";
 const STATE_DOWNLOADING     = "downloading";
@@ -105,16 +108,18 @@ const WRITE_ERROR_DELETE_FILE           
 const WRITE_ERROR_OPEN_PATCH_FILE          = 63;
 const WRITE_ERROR_PATCH_FILE               = 64;
 const WRITE_ERROR_APPLY_DIR_PATH           = 65;
 const WRITE_ERROR_CALLBACK_PATH            = 66;
 const WRITE_ERROR_FILE_ACCESS_DENIED       = 67;
 const WRITE_ERROR_DIR_ACCESS_DENIED        = 68;
 const WRITE_ERROR_DELETE_BACKUP            = 69;
 const WRITE_ERROR_EXTRACT                  = 70;
+const CALLBACK_ERROR_WAIT_PID              = 81;
+const CALLBACK_ERROR_SHARING_VIOLATION     = 82;
 
 // Array of write errors to simplify checks for write errors
 const WRITE_ERRORS = [WRITE_ERROR,
                       WRITE_ERROR_ACCESS_DENIED,
                       WRITE_ERROR_CALLBACK_APP,
                       WRITE_ERROR_FILE_COPY,
                       WRITE_ERROR_DELETE_FILE,
                       WRITE_ERROR_OPEN_PATCH_FILE,
@@ -168,16 +173,21 @@ const DEFAULT_SOCKET_MAX_ERRORS = 10;
 
 // The number of milliseconds to wait before retrying a connection error.
 const DEFAULT_SOCKET_RETRYTIMEOUT = 2000;
 
 // Default maximum number of elevation cancelations per update version before
 // giving up.
 const DEFAULT_CANCELATIONS_OSX_MAX = 3;
 
+// When an update fails because files are in use, we eventually want to force an
+// update. This value specifies the delay between the first failure and the
+// decision to force the next update (in milliseconds).
+const SHARING_VIOLATION_FORCE_DELAY = 24 * 60 * 60 * 1000;  // 24 hours
+
 // This maps app IDs to their respective notification topic which signals when
 // the application's user interface has been displayed.
 const APPID_TO_TOPIC = {
   // Firefox
   "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}": "sessionstore-windows-restored",
   // SeaMonkey
   "{92650c4d-4b8e-4d2a-b7eb-24ecf4f6b63a}": "sessionstore-windows-restored",
   // Thunderbird
@@ -860,16 +870,21 @@ function cleanupActiveUpdate() {
   // Move the update from the Active Update list into the Past Updates list.
   var um = Cc["@mozilla.org/updates/update-manager;1"].
            getService(Ci.nsIUpdateManager);
   // Setting |activeUpdate| to null will move the active update to the update
   // history.
   um.activeUpdate = null;
   um.saveUpdates();
 
+  // Unset prefs indicating previous install failure now that the install has
+  // succeeded
+  Services.prefs.clearUserPref(PREF_APP_UPDATE_IN_USE_FAIL_TIME);
+  Services.prefs.clearUserPref(PREF_APP_UPDATE_IN_USE_FAIL_VERSION);
+
   // Now trash the updates directory, since we're done with it
   cleanUpUpdatesDir();
 }
 
 /**
  * An enumeration of items in a JS array.
  * @constructor
  */
@@ -1021,16 +1036,54 @@ function handleUpdateFailure(update, err
       Services.prefs.setIntPref(PREF_APP_UPDATE_SERVICE_ERRORS,
                                 failCount);
     }
 
     writeStatusFile(getUpdatesDir(), update.state = STATE_PENDING);
     return true;
   }
 
+  if (update.errorCode == CALLBACK_ERROR_WAIT_PID ||
+      update.errorCode == CALLBACK_ERROR_SHARING_VIOLATION) {
+    update.statusText = gUpdateBundle.GetStringFromName("appInUseFailure");
+    // Since staging won't work successfully when the process is in use don't
+    // try to perform a replace request.
+    // Comment needs to go with the force code.
+    writeStatusFile(getUpdatesDir(), update.state = STATE_PENDING);
+
+    let previousFailTime = Services.prefs.getIntPref(PREF_APP_UPDATE_IN_USE_FAIL_TIME, null);
+    let previousFailVersion = Services.prefs.getCharPref(PREF_APP_UPDATE_IN_USE_FAIL_VERSION, "");
+    if (previousFailVersion == update.appVersion) {
+      if (Date.now() - previousFailTime >= SHARING_VIOLATION_FORCE_DELAY) {
+        let updateDir;
+        try {
+          updateDir = getUpdatesDir();
+        } catch (e) {
+          LOG("handleUpdateFailure - unable to get the updates patch " +
+              "directory. Exception: " + e);
+        }
+        if (updateDir) {
+          let forceFile = updateDir;
+          forceFile.append(FILE_UPDATE_FORCE);
+          forceFile.create();
+        }
+      }
+    } else {
+      // This is the first failure we have experienced from this cause, for this
+      // update. Keep a record of the time it happened in these prefs.
+      Services.prefs.setIntPref(PREF_APP_UPDATE_IN_USE_FAIL_TIME, Date.now());
+      Services.prefs.setCharPref(PREF_APP_UPDATE_IN_USE_FAIL_VERSION,
+                                 update.appVersion);
+    }
+
+    let prompter = Cc["@mozilla.org/updates/update-prompt;1"].
+                   createInstance(Ci.nsIUpdatePrompt);
+    prompter.showUpdateError(update);
+  }
+
   if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_SERVICE_ERRORS)) {
     Services.prefs.clearUserPref(PREF_APP_UPDATE_SERVICE_ERRORS);
   }
 
   return false;
 }
 
 /**

--- a/toolkit/mozapps/update/tests/data/sharedUpdateXML.js
+++ b/toolkit/mozapps/update/tests/data/sharedUpdateXML.js
@@ -39,16 +39,18 @@ const SERVICE_INVALID_APPLYTO_DIR_ERROR 
 const SERVICE_INVALID_INSTALL_DIR_PATH_ERROR   = 55;
 const SERVICE_INVALID_WORKING_DIR_PATH_ERROR   = 56;
 const INVALID_APPLYTO_DIR_STAGED_ERROR         = 72;
 const INVALID_APPLYTO_DIR_ERROR                = 74;
 const INVALID_INSTALL_DIR_PATH_ERROR           = 75;
 const INVALID_WORKING_DIR_PATH_ERROR           = 76;
 const INVALID_CALLBACK_PATH_ERROR              = 77;
 const INVALID_CALLBACK_DIR_ERROR               = 78;
+const CALLBACK_ERROR_WAIT_PID                  = 81;
+const CALLBACK_ERROR_SHARING_VIOLATION         = 82;
 
 // Error codes 80 through 99 are reserved for nsUpdateService.js and are not
 // defined in common/errors.h
 const ERR_OLDER_VERSION_OR_SAME_BUILD      = 90;
 const ERR_UPDATE_STATE_NONE                = 91;
 const ERR_CHANNEL_CHANGE                   = 92;
 
 const STATE_FAILED_DELIMETER = ": ";
@@ -60,16 +62,20 @@ const STATE_FAILED_CRC_ERROR =
 const STATE_FAILED_READ_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + READ_ERROR;
 const STATE_FAILED_WRITE_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + WRITE_ERROR;
 const STATE_FAILED_MAR_CHANNEL_MISMATCH_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + MAR_CHANNEL_MISMATCH_ERROR;
 const STATE_FAILED_VERSION_DOWNGRADE_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + VERSION_DOWNGRADE_ERROR;
+const STATE_FAILED_CALLBACK_ERROR_WAIT_PID =
+  STATE_FAILED + STATE_FAILED_DELIMETER + CALLBACK_ERROR_WAIT_PID;
+const STATE_FAILED_CALLBACK_ERROR_SHARING_VIOLATION =
+  STATE_FAILED + STATE_FAILED_DELIMETER + CALLBACK_ERROR_SHARING_VIOLATION;
 const STATE_FAILED_SERVICE_COULD_NOT_COPY_UPDATER =
   STATE_FAILED + STATE_FAILED_DELIMETER + SERVICE_COULD_NOT_COPY_UPDATER;
 const STATE_FAILED_SERVICE_INVALID_APPLYTO_DIR_STAGED_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + SERVICE_INVALID_APPLYTO_DIR_STAGED_ERROR;
 const STATE_FAILED_SERVICE_INVALID_APPLYTO_DIR_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + SERVICE_INVALID_APPLYTO_DIR_ERROR;
 const STATE_FAILED_SERVICE_INVALID_INSTALL_DIR_PATH_ERROR =
   STATE_FAILED + STATE_FAILED_DELIMETER + SERVICE_INVALID_INSTALL_DIR_PATH_ERROR;

--- a/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js
+++ b/toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js
@@ -89,17 +89,21 @@ const CALL_QUIT = "calling QuitProgressU
 const ERR_UPDATE_IN_PROGRESS = "Update already in progress! Exiting";
 const ERR_RENAME_FILE = "rename_file: failed to rename file";
 const ERR_ENSURE_COPY = "ensure_copy: failed to copy the file";
 const ERR_UNABLE_OPEN_DEST = "unable to open destination file";
 const ERR_BACKUP_DISCARD = "backup_discard: unable to remove";
 const ERR_MOVE_DESTDIR_7 = "Moving destDir to tmpDir failed, err: 7";
 const ERR_BACKUP_CREATE_7 = "backup_create failed: 7";
 const ERR_LOADSOURCEFILE_FAILED = "LoadSourceFile failed";
-const ERR_PARENT_PID_PERSISTS = "The parent process didn't exit! Continuing with update.";
+
+const ERR_CALLBACK_OPEN = "NS_main: callback app file open attempt";
+const ERR_CALLBACK_IN_USE = "NS_main: callback app file in use, failed to exclusively open executable file:";
+const ERR_NO_PID_EXIT = "The parent process didn't exit and the update is not being forced.";
+const ERR_PARENT_PID_PERSISTS = "The parent process didn't exit, but the update is being forced.";
 
 const LOG_SVC_SUCCESSFUL_LAUNCH = "Process was started... waiting on result.";
 
 // Typical end of a message when calling assert
 const MSG_SHOULD_EQUAL = " should equal the expected value";
 const MSG_SHOULD_EXIST = "the file or directory should exist";
 const MSG_SHOULD_NOT_EXIST = "the file or directory should not exist";
 
@@ -3104,19 +3108,21 @@ function checkUpdateLogContents(aCompare
  *
  * @param   aCheckString
  *          The string to check if the update log contains.
  */
 function checkUpdateLogContains(aCheckString) {
   let updateLog = getUpdateLog(FILE_LAST_UPDATE_LOG);
   let updateLogContents = readFileBytes(updateLog).replace(/\r\n/g, "\n");
   updateLogContents = replaceLogPaths(updateLogContents);
-  Assert.notEqual(updateLogContents.indexOf(aCheckString), -1,
-                  "the update log contents should contain value: " +
-                  aCheckString);
+  if (updateLogContents.indexOf(aCheckString) == -1) {
+    logTestInfo(updateLogContents);
+    Assert.ok(false,
+              `The update log contents should contain value: ${aCheckString}`);
+  }
 }
 
 /**
  * Helper function for updater binary tests for verifying the state of files and
  * directories after a successful update.
  *
  * @param   aGetFileFunc
  *          The function used to get the files in the directory to be checked.

--- a/toolkit/mozapps/update/tests/unit_base_updater/marAppInUseFailureComplete_win.js
+++ b/toolkit/mozapps/update/tests/unit_base_updater/marAppInUseFailureComplete_win.js
@@ -5,58 +5,52 @@
 /* Application in use complete MAR file patch apply success test */
 
 function run_test() {
   if (!setupTestCommon()) {
     return;
   }
   gTestFiles = gTestFilesCompleteSuccess;
   gTestDirs = gTestDirsCompleteSuccess;
+  setTestFilesAndDirsForFailure();
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
   runHelperFileInUse(DIR_RESOURCES + gCallbackBinFile, false);
 }
 
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
-  runUpdate(STATE_SUCCEEDED, false, 0, true);
+  runUpdate(STATE_FAILED_CALLBACK_ERROR_SHARING_VIOLATION, false, 1, true);
 }
 
 /**
  * Called after the call to runUpdate finishes.
  */
 function runUpdateFinished() {
   waitForHelperExit();
 }
 
 /**
  * Called after the call to waitForHelperExit finishes.
  */
 function waitForHelperExitFinished() {
-  checkPostUpdateAppLog();
-}
-
-/**
- * Called after the call to checkPostUpdateAppLog finishes.
- */
-function checkPostUpdateAppLogFinished() {
-  checkAppBundleModTime();
   standardInit();
-  checkPostUpdateRunningFile(true);
-  checkFilesAfterUpdateSuccess(getApplyDirFile);
-  checkUpdateLogContents(LOG_COMPLETE_SUCCESS);
+  checkPostUpdateRunningFile(false);
+  checkFilesAfterUpdateFailure(getApplyDirFile);
+  checkUpdateLogContains(ERR_CALLBACK_OPEN);
+  checkUpdateLogContains(ERR_CALLBACK_IN_USE);
   do_execute_soon(waitForUpdateXMLFiles);
 }
 
 /**
  * Called after the call to waitForUpdateXMLFiles finishes.
  */
 function waitForUpdateXMLFilesFinished() {
-  checkUpdateManager(STATE_NONE, false, STATE_SUCCEEDED, 0, 1);
+  checkUpdateManager(STATE_NONE, false, STATE_PENDING, CALLBACK_ERROR_SHARING_VIOLATION, 1);
   checkCallbackLog();
 }

--- a/toolkit/mozapps/update/tests/unit_base_updater/marAppInUseForceSuccessComplete_win.js
+++ b/toolkit/mozapps/update/tests/unit_base_updater/marAppInUseForceSuccessComplete_win.js
@@ -12,16 +12,19 @@ function run_test() {
   gTestDirs = gTestDirsCompleteSuccess;
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
+  let forceFile = getUpdatesPatchDir();
+  forceFile.append("update.force");
+  forceFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);
   runHelperFileInUse(DIR_RESOURCES + gCallbackBinFile, false);
 }
 
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
   runUpdate(STATE_SUCCEEDED, false, 0, true);

--- a/toolkit/mozapps/update/tests/unit_base_updater/marPIDPersistsFailureComplete_win.js
+++ b/toolkit/mozapps/update/tests/unit_base_updater/marPIDPersistsFailureComplete_win.js
@@ -5,16 +5,17 @@
 /* Application in use complete MAR file patch apply success test */
 
 function run_test() {
   if (!setupTestCommon()) {
     return;
   }
   gTestFiles = gTestFilesCompleteSuccess;
   gTestDirs = gTestDirsCompleteSuccess;
+  setTestFilesAndDirsForFailure();
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
   runHelperPIDPersists(DIR_RESOURCES + gCallbackBinFile, false);
@@ -23,43 +24,36 @@ function setupUpdaterTestFinished() {
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
   if (!gPIDPersistProcess.pid) {
     do_execute_soon(waitForHelperSleepFinished);
     return;
   }
-  runUpdate(STATE_SUCCEEDED, false, 0, true);
+  runUpdate(STATE_FAILED_CALLBACK_ERROR_WAIT_PID, false, 1, true);
 }
 
 /**
  * Called after the call to runUpdate finishes.
  */
 function runUpdateFinished() {
   waitForHelperExit();
 }
 
 /**
  * Called after the call to waitForHelperExit finishes.
  */
 function waitForHelperExitFinished() {
-  checkPostUpdateAppLog();
-}
-
-/**
- * Called after the call to checkPostUpdateAppLog finishes.
- */
-function checkPostUpdateAppLogFinished() {
   standardInit();
-  checkPostUpdateRunningFile(true);
-  checkFilesAfterUpdateSuccess(getApplyDirFile);
-  checkUpdateLogContains(ERR_PARENT_PID_PERSISTS);
+  checkPostUpdateRunningFile(false);
+  checkFilesAfterUpdateFailure(getApplyDirFile);
+  checkUpdateLogContains(ERR_NO_PID_EXIT);
   do_execute_soon(waitForUpdateXMLFiles);
 }
 
 /**
  * Called after the call to waitForUpdateXMLFiles finishes.
  */
 function waitForUpdateXMLFilesFinished() {
-  checkUpdateManager(STATE_NONE, false, STATE_SUCCEEDED, 0, 1);
+  checkUpdateManager(STATE_NONE, false, STATE_PENDING, CALLBACK_ERROR_WAIT_PID, 1);
   checkCallbackLog();
 }

--- a/toolkit/mozapps/update/tests/unit_base_updater/marPIDPersistsForceSuccessComplete_win.js
+++ b/toolkit/mozapps/update/tests/unit_base_updater/marPIDPersistsForceSuccessComplete_win.js
@@ -12,16 +12,19 @@ function run_test() {
   gTestDirs = gTestDirsCompleteSuccess;
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
+  let forceFile = getUpdatesPatchDir();
+  forceFile.append("update.force");
+  forceFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);
   runHelperPIDPersists(DIR_RESOURCES + gCallbackBinFile, false);
 }
 
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
   if (!gPIDPersistProcess.pid) {

--- a/toolkit/mozapps/update/tests/unit_base_updater/xpcshell.ini
+++ b/toolkit/mozapps/update/tests/unit_base_updater/xpcshell.ini
@@ -40,17 +40,22 @@ reason = Windows only test
 skip-if = os != 'win'
 reason = Windows only test
 [marCallbackAppStageSuccessComplete_win.js]
 skip-if = os != 'win'
 reason = Windows only test
 [marCallbackAppStageSuccessPartial_win.js]
 skip-if = os != 'win'
 reason = Windows only test
-[marAppInUseSuccessComplete.js]
+[marAppInUseForceSuccessComplete_win.js]
+skip-if = os != 'win'
+reason = Windows only test
+[marAppInUseFailureComplete_win.js]
+skip-if = os != 'win'
+reason = Windows only test
 [marAppInUseStageSuccessComplete_unix.js]
 skip-if = os == 'win'
 reason = not a Windows test
 [marAppInUseStageFailureComplete_win.js]
 skip-if = os != 'win'
 reason = Windows only test
 [marFileLockedFailureComplete_win.js]
 skip-if = os != 'win'
@@ -83,17 +88,20 @@ reason = Windows only test
 skip-if = os != 'win'
 reason = Windows only test
 [marRMRFDirFileInUseStageFailureComplete_win.js]
 skip-if = os != 'win'
 reason = Windows only test
 [marRMRFDirFileInUseStageFailurePartial_win.js]
 skip-if = os != 'win'
 reason = Windows only test
-[marPIDPersistsSuccessComplete_win.js]
+[marPIDPersistsFailureComplete_win.js]
+skip-if = os != 'win'
+reason = Windows only test
+[marPIDPersistsForceSuccessComplete_win.js]
 skip-if = os != 'win'
 reason = Windows only test
 [marAppApplyDirLockedStageFailure_win.js]
 skip-if = os != 'win'
 reason = Windows only test
 [marAppApplyUpdateAppBinInUseStageSuccess_win.js]
 skip-if = os != 'win'
 reason = Windows only test

--- a/toolkit/mozapps/update/tests/unit_service_updater/marAppInUseFailureCompleteSvc_win.js
+++ b/toolkit/mozapps/update/tests/unit_service_updater/marAppInUseFailureCompleteSvc_win.js
@@ -5,58 +5,52 @@
 /* Application in use complete MAR file patch apply success test */
 
 function run_test() {
   if (!setupTestCommon()) {
     return;
   }
   gTestFiles = gTestFilesCompleteSuccess;
   gTestDirs = gTestDirsCompleteSuccess;
+  setTestFilesAndDirsForFailure();
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
   runHelperFileInUse(DIR_RESOURCES + gCallbackBinFile, false);
 }
 
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
-  runUpdate(STATE_SUCCEEDED, false, 0, true);
+  runUpdate(STATE_FAILED_CALLBACK_ERROR_SHARING_VIOLATION, false, 1, true);
 }
 
 /**
  * Called after the call to runUpdate finishes.
  */
 function runUpdateFinished() {
   waitForHelperExit();
 }
 
 /**
  * Called after the call to waitForHelperExit finishes.
  */
 function waitForHelperExitFinished() {
-  checkPostUpdateAppLog();
-}
-
-/**
- * Called after the call to checkPostUpdateAppLog finishes.
- */
-function checkPostUpdateAppLogFinished() {
-  checkAppBundleModTime();
   standardInit();
-  checkPostUpdateRunningFile(true);
-  checkFilesAfterUpdateSuccess(getApplyDirFile);
-  checkUpdateLogContents(LOG_COMPLETE_SUCCESS);
+  checkPostUpdateRunningFile(false);
+  checkFilesAfterUpdateFailure(getApplyDirFile);
+  checkUpdateLogContains(ERR_CALLBACK_OPEN);
+  checkUpdateLogContains(ERR_CALLBACK_IN_USE);
   do_execute_soon(waitForUpdateXMLFiles);
 }
 
 /**
  * Called after the call to waitForUpdateXMLFiles finishes.
  */
 function waitForUpdateXMLFilesFinished() {
-  checkUpdateManager(STATE_NONE, false, STATE_SUCCEEDED, 0, 1);
+  checkUpdateManager(STATE_NONE, false, STATE_PENDING, CALLBACK_ERROR_SHARING_VIOLATION, 1);
   checkCallbackLog();
 }

--- a/toolkit/mozapps/update/tests/unit_service_updater/marAppInUseForceSuccessCompleteSvc_win.js
+++ b/toolkit/mozapps/update/tests/unit_service_updater/marAppInUseForceSuccessCompleteSvc_win.js
@@ -12,16 +12,19 @@ function run_test() {
   gTestDirs = gTestDirsCompleteSuccess;
   setupUpdaterTest(FILE_COMPLETE_MAR, false);
 }
 
 /**
  * Called after the call to setupUpdaterTest finishes.
  */
 function setupUpdaterTestFinished() {
+  let forceFile = getUpdatesPatchDir();
+  forceFile.append("update.force");
+  forceFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);
   runHelperFileInUse(DIR_RESOURCES + gCallbackBinFile, false);
 }
 
 /**
  * Called after the call to waitForHelperSleep finishes.
  */
 function waitForHelperSleepFinished() {
   runUpdate(STATE_SUCCEEDED, false, 0, true);

--- a/toolkit/mozapps/update/tests/unit_service_updater/xpcshell.ini
+++ b/toolkit/mozapps/update/tests/unit_service_updater/xpcshell.ini
@@ -41,17 +41,19 @@ run-sequentially = Uses the Mozilla Main
 [marCallbackAppSuccessCompleteSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marCallbackAppSuccessPartialSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marCallbackAppStageSuccessCompleteSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marCallbackAppStageSuccessPartialSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
-[marAppInUseSuccessCompleteSvc.js]
+[marAppInUseForceSuccessCompleteSvc_win.js]
+run-sequentially = Uses the Mozilla Maintenance Service.
+[marAppInUseFailureCompleteSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marAppInUseStageFailureCompleteSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marFileLockedFailureCompleteSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marFileLockedFailurePartialSvc_win.js]
 run-sequentially = Uses the Mozilla Maintenance Service.
 [marFileLockedStageFailureCompleteSvc_win.js]

--- a/toolkit/mozapps/update/updater/updater.cpp
+++ b/toolkit/mozapps/update/updater/updater.cpp
@@ -2953,36 +2953,53 @@ int NS_main(int argc, NS_tchar **argv)
            "a child of the installation directory! Exiting."));
       LogFinish();
       return 1;
     }
   }
 #endif
 
 #ifdef XP_WIN
+  bool forceUpdate = false;
+  NS_tchar forceFile[MAXPATHLEN];
+  NS_tsnprintf(forceFile, sizeof(forceFile)/sizeof(forceFile[0]),
+               NS_T("%s/update.force"), gPatchDirPath);
+  if (!NS_taccess(forceFile, F_OK)) {
+    forceUpdate = true;
+  }
+
   if (pid > 0) {
     HANDLE parent = OpenProcess(SYNCHRONIZE, false, (DWORD) pid);
     // May return nullptr if the parent process has already gone away.
     // Otherwise, wait for the parent process to exit before starting the
     // update.
     if (parent) {
       DWORD waitTime = PARENT_WAIT;
 #ifdef TEST_UPDATER
       if (EnvHasValue("MOZ_TEST_SHORTER_WAIT_PID")) {
         // Use a shorter time to wait for the PID to exit for the test.
         waitTime = 100;
       }
 #endif
       DWORD result = WaitForSingleObject(parent, waitTime);
       CloseHandle(parent);
       if (result != WAIT_OBJECT_0) {
-        // Continue to update since the parent application sometimes doesn't
-        // exit (see bug 1375242) so any fixes to the parent application will
-        // be applied instead of leaving the client in a broken state.
-        LOG(("The parent process didn't exit! Continuing with update."));
+        if (forceUpdate) {
+          LOG(("The parent process didn't exit, but the update is being forced."));
+        } else {
+          LOG(("The parent process didn't exit and the update is not being forced."));
+          LogFinish();
+          WriteStatusFile(CALLBACK_ERROR_WAIT_PID);
+
+          LaunchCallbackApp(argv[5],
+                            argc - callbackIndex,
+                            argv + callbackIndex,
+                            sUsingService);
+          return 1;
+        }
       }
     }
   }
 #else
   if (pid > 0)
     waitpid(pid, nullptr, 0);
 #endif
 
@@ -3499,22 +3516,27 @@ int NS_main(int argc, NS_tchar **argv)
              "File: " LOG_S ". Last error: %d", retries,
              targetPath, lastWriteError));
 
         Sleep(100);
       } while (++retries <= max_retries);
 
       // CreateFileW will fail if the callback executable is already in use.
       if (callbackFile == INVALID_HANDLE_VALUE) {
-        // Only fail the update if the last error was not a sharing violation.
-        if (lastWriteError != ERROR_SHARING_VIOLATION) {
+        // If CreateFileW failed, the update must fail with one exception:
+        // If the error is a sharing violation and we are forcing the update
+        // despite the sharing violation.
+        if (!(lastWriteError == ERROR_SHARING_VIOLATION && forceUpdate)) {
           LOG(("NS_main: callback app file in use, failed to exclusively open " \
-               "executable file: " LOG_S, argv[callbackIndex]));
+               "executable file: " LOG_S ", err: %d", argv[callbackIndex],
+               lastWriteError));
           LogFinish();
-          if (lastWriteError == ERROR_ACCESS_DENIED) {
+          if (lastWriteError == ERROR_SHARING_VIOLATION) {
+            WriteStatusFile(CALLBACK_ERROR_SHARING_VIOLATION);
+          } else if (lastWriteError == ERROR_ACCESS_DENIED) {
             WriteStatusFile(WRITE_ERROR_ACCESS_DENIED);
           } else {
             WriteStatusFile(WRITE_ERROR_CALLBACK_APP);
           }
 
           NS_tremove(gCallbackBackupPath);
           EXIT_WHEN_ELEVATED(elevatedLockFilePath, updateLockFileHandle, 1);
           LaunchCallbackApp(argv[5],