--- a/testing/xpcshell/head.js
+++ b/testing/xpcshell/head.js
@@ -1102,16 +1102,26 @@ function do_get_profile(notifyProfileAft
       }
       return null;
     },
     QueryInterface: _XPCOMUtils.generateQI(["nsIDirectoryServiceProvider"]),
   };
   _Services.dirsvc.QueryInterface(Ci.nsIDirectoryService)
            .registerProvider(provider);
 
+  try {
+    _Services.dirsvc.undefine("TmpD");
+  } catch (e) {
+    // This throws if the key is not already registers, but that
+    // doesn't matter.
+    if (e.result != Cr.NS_ERROR_FAILURE) {
+      throw e;
+    }
+  }
+
   // We need to update the crash events directory when the profile changes.
   if (runningInParent &&
       "@mozilla.org/toolkit/crash-reporter;1" in Cc) {
     let crashReporter =
         Cc["@mozilla.org/toolkit/crash-reporter;1"]
           .getService(Ci.nsICrashReporter);
     crashReporter.UpdateCrashEventsDir();
   }

--- a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
@@ -26,16 +26,19 @@ const {OS} = ChromeUtils.import("resourc
 
 ChromeUtils.defineModuleGetter(this, "Extension",
                                "resource://gre/modules/Extension.jsm");
 XPCOMUtils.defineLazyGetter(this, "Management", () => {
   let {Management} = ChromeUtils.import("resource://gre/modules/Extension.jsm", {});
   return Management;
 });
 
+ChromeUtils.defineModuleGetter(this, "FileTestUtils",
+                               "resource://testing-common/FileTestUtils.jsm");
+
 XPCOMUtils.defineLazyServiceGetter(this, "aomStartup",
                                    "@mozilla.org/addons/addon-manager-startup;1",
                                    "amIAddonManagerStartup");
 XPCOMUtils.defineLazyServiceGetter(this, "rdfService",
                                    "@mozilla.org/rdf/rdf-service;1", "nsIRDFService");
 XPCOMUtils.defineLazyServiceGetter(this, "uuidGen",
                                    "@mozilla.org/uuid-generator;1", "nsIUUIDGenerator");
 
@@ -290,22 +293,37 @@ var AddonTestUtils = {
       if (file.exists()) {
         throw new Error(`Test cleanup: path ${file.path} exists when it should not`);
       }
     }
 
     testScope.registerCleanupFunction(() => {
       this.cleanupTempXPIs();
 
+      let ignoreEntries = new Set();
+      {
+        // FileTestUtils lazily creates a directory to hold the temporary files
+        // it creates. If that directory exists, ignore it.
+        let {value} = Object.getOwnPropertyDescriptor(FileTestUtils,
+                                                      "_globalTemporaryDirectory");
+        if (value) {
+          ignoreEntries.add(value.leafName);
+        }
+      }
+
       // Check that the temporary directory is empty
       var dirEntries = this.tempDir.directoryEntries
                            .QueryInterface(Ci.nsIDirectoryEnumerator);
       var entries = [];
-      while (dirEntries.hasMoreElements())
-        entries.push(dirEntries.nextFile.leafName);
+      while (dirEntries.hasMoreElements()) {
+        let {leafName} = dirEntries.nextFile;
+        if (!ignoreEntries.has(leafName)) {
+          entries.push(leafName);
+        }
+      }
       if (entries.length)
         throw new Error(`Found unexpected files in temporary directory: ${entries.join(", ")}`);
 
       dirEntries.close();
 
       try {
         appDirForAddons.remove(true);
       } catch (ex) {
@@ -1049,16 +1067,26 @@ var AddonTestUtils = {
         if (prop == key)
           return dir.clone();
         return null;
       },
 
       QueryInterface: XPCOMUtils.generateQI([Ci.nsIDirectoryServiceProvider]),
     };
     Services.dirsvc.registerProvider(dirProvider);
+
+    try {
+      Services.dirsvc.undefine(key);
+    } catch (e) {
+      // This throws if the key is not already registers, but that
+      // doesn't matter.
+      if (e.result != Cr.NS_ERROR_FAILURE) {
+        throw e;
+      }
+    }
   },
 
   /**
    * Returns a promise that resolves when the given add-on event is fired. The
    * resolved value is an array of arguments passed for the event.
    *
    * @param {string} event
    *        The name of the AddonListener event handler method for which

--- a/toolkit/mozapps/extensions/internal/XPIInstall.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIInstall.jsm
@@ -1388,16 +1388,28 @@ class AddonInstall {
       // Installation is already running
       return;
     default:
       throw new Error("Cannot start installing from this state");
     }
   }
 
   /**
+   * Called during XPIProvider shutdown so that we can do any necessary
+   * pre-shutdown cleanup.
+   */
+  onShutdown() {
+    switch (this.state) {
+    case AddonManager.STATE_POSTPONED:
+      this.removeTemporaryFile();
+      break;
+    }
+  }
+
+  /**
    * Cancels installation of this add-on.
    *
    * Note this method is overridden to handle additional state in
    * the subclass DownloadAddonInstall.
    *
    * @throws if installation cannot be cancelled from the current state
    */
   cancel() {

--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -2321,16 +2321,22 @@ var XPIProvider = {
     logger.debug("shutdown");
 
     // Stop anything we were doing asynchronously
     this.cancelAll();
 
     this.activeAddons.clear();
     this.allAppGlobal = true;
 
+    for (let install of this.installs) {
+      if (install.onShutdown()) {
+        install.onShutdown();
+      }
+    }
+
     // If there are pending operations then we must update the list of active
     // add-ons
     if (Services.prefs.getBoolPref(PREF_PENDING_OPERATIONS, false)) {
       AddonManagerPrivate.recordSimpleMeasure("XPIDB_pending_ops", 1);
       XPIDatabase.updateActiveAddons();
       Services.prefs.setBoolPref(PREF_PENDING_OPERATIONS, false);
     }
 

new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_harness.js
@@ -0,0 +1,11 @@
+"use strict";
+
+// Test that the test harness is sane.
+
+// Test that the temporary directory is actually overridden in the
+// directory service.
+add_task(async function test_TmpD_override() {
+  equal(FileUtils.getDir("TmpD", []).path,
+        AddonTestUtils.tempDir.path,
+        "Should get the correct temporary directory from the directory service");
+});

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_install_syntax_error.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_install_syntax_error.js
@@ -33,16 +33,17 @@ add_task(async function install_xpi() {
   Assert.equal(install1.error, AddonManager.ERROR_CORRUPT_FILE);
 
   // Replace xpi1 with xpi2 to have the same filename to reproduce install error
   xpi2.moveTo(xpi1.parent, xpi1.leafName);
 
   let install2 = await AddonManager.getInstallForFile(xpi2);
   Assert.notEqual(install2.error, AddonManager.ERROR_CORRUPT_FILE);
 
+  xpi1.remove(false);
 });
 
 function run_test() {
   createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1");
   startupManager();
 
   run_next_test();
 }

--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
@@ -12,16 +12,17 @@ support-files =
 tags = blocklist
 [test_blocklist_gfx.js]
 tags = blocklist
 [test_cache_certdb.js]
 run-if = addon_signing
 [test_cacheflush.js]
 [test_gmpProvider.js]
 skip-if = appname != "firefox"
+[test_harness.js]
 [test_invalid_install_rdf.js]
 [test_isReady.js]
 [test_pluginInfoURL.js]
 tags = blocklist
 [test_provider_markSafe.js]
 [test_provider_shutdown.js]
 [test_provider_unsafe_access_shutdown.js]
 [test_provider_unsafe_access_startup.js]