diff --git a/browser/app/profile/firefox.js b/browser/app/profile/firefox.js
index c2083d00e3e3..094c7b1adaf9 100644
--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -69,6 +69,8 @@ pref("extensions.screenshots.system-disabled", true);
 // Disable add-ons that are not installed by the user in all scopes by default.
 // See the SCOPE constants in AddonManager.jsm for values to use here.
 pref("extensions.autoDisableScopes", 15);
+// Scopes to scan for changes at startup.
+pref("extensions.startupScanScopes", 0);
 
 // This is where the profiler WebExtension API will look for breakpad symbols.
 // NOTE: deliberately http right now since https://symbols.mozilla.org is not supported.
diff --git a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
index 8852baa93bdd..a103a713b583 100644
--- a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
@@ -243,6 +243,9 @@ var AddonTestUtils = {
     // By default don't disable add-ons from any scope
     Services.prefs.setIntPref("extensions.autoDisableScopes", 0);
 
+    // And scan for changes at startup
+    Services.prefs.setIntPref("extensions.startupScanScopes", 15);
+
     // By default, don't cache add-ons in AddonRepository.jsm
     Services.prefs.setBoolPref("extensions.getAddons.cache.enabled", false);
 
diff --git a/toolkit/mozapps/extensions/internal/XPIProvider.jsm b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
index c864958684b0..6a22f5b80819 100644
--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -117,6 +117,7 @@ const PREF_EM_UPDATE_BACKGROUND_URL   = "extensions.update.background.url";
 const PREF_EM_ENABLED_ADDONS          = "extensions.enabledAddons";
 const PREF_EM_EXTENSION_FORMAT        = "extensions.";
 const PREF_EM_ENABLED_SCOPES          = "extensions.enabledScopes";
+const PREF_EM_STARTUP_SCAN_SCOPES     = "extensions.startupScanScopes";
 const PREF_EM_SHOW_MISMATCH_UI        = "extensions.showMismatchUI";
 const PREF_XPI_ENABLED                = "xpinstall.enabled";
 const PREF_XPI_WHITELIST_REQUIRED     = "xpinstall.whitelist.required";
@@ -327,6 +328,9 @@ const LAZY_OBJECTS = ["XPIDatabase", "XPIDatabaseReconcile"];
 
 var gLazyObjectsLoaded = false;
 
+XPCOMUtils.defineLazyPreferenceGetter(this, "gStartupScanScopes",
+                                      PREF_EM_STARTUP_SCAN_SCOPES, 0);
+
 function loadLazyObjects() {
   let uri = "resource://gre/modules/addons/XPIProviderUtils.js";
   let scope = Cu.Sandbox(Services.scriptSecurityManager.getSystemPrincipal(), {
@@ -2322,19 +2326,38 @@ this.XPIStates = {
   /**
    * Walk through all install locations, highest priority first,
    * comparing the on-disk state of extensions to what is stored in prefs.
+   *
+   * @param {bool} [ignoreSideloads = true]
+   *        If true, ignore changes in scopes where we don't accept
+   *        side-loads.
+   *
    * @return true if anything has changed.
    */
-  getInstallState() {
+  getInstallState(ignoreSideloads = true) {
     let oldState = this.loadExtensionState();
     let changed = false;
     this.db = new SerializableMap();
 
     for (let location of XPIProvider.installLocations) {
-      // The list of add-on like file/directory names in the install location.
-      let addons = location.getAddonLocations();
       // The results of scanning this location.
       let foundAddons = new SerializableMap();
 
+      // Don't bother checking scopes where we don't accept side-loads.
+      if (ignoreSideloads && !(location.scope & gStartupScanScopes)) {
+        if (location.name in oldState) {
+          for (let [id, state] of Object.entries(oldState[location.name])) {
+            foundAddons.set(id, new XPIState(state));
+          }
+
+          this.db.set(location.name, foundAddons);
+          delete oldState[location.name];
+        }
+        continue;
+      }
+
+      // The list of add-on like file/directory names in the install location.
+      let addons = location.getAddonLocations(!ignoreSideloads);
+
       // What our old state thinks should be in this location.
       let locState = {};
       if (location.name in oldState) {
@@ -3762,8 +3785,7 @@ this.XPIProvider = {
    *         if it is a new profile or the version is unknown
    * @return true if a change requiring a restart was detected
    */
-  checkForChanges(aAppChanged, aOldAppVersion,
-                                                aOldPlatformVersion) {
+  checkForChanges(aAppChanged, aOldAppVersion, aOldPlatformVersion) {
     logger.debug("checkForChanges");
 
     // Keep track of whether and why we need to open and update the database at
@@ -3810,7 +3832,7 @@ this.XPIProvider = {
 
     // Telemetry probe added around getInstallState() to check perf
     let telemetryCaptureTime = Cu.now();
-    let installChanged = XPIStates.getInstallState();
+    let installChanged = XPIStates.getInstallState(aAppChanged === false);
     let telemetry = Services.telemetry;
     telemetry.getHistogramById("CHECK_ADDONS_MODIFIED_MS").add(Math.round(Cu.now() - telemetryCaptureTime));
     if (installChanged) {
@@ -8073,7 +8095,7 @@ class DirectoryInstallLocation {
     if (!aDirectory.isDirectory())
       throw new Error("Location must be a directory.");
 
-    this._readAddons();
+    this.initialized = false;
   }
 
   /**
@@ -8137,7 +8159,12 @@ class DirectoryInstallLocation {
   /**
    * Finds all the add-ons installed in this location.
    */
-  _readAddons() {
+  _readAddons(rescan = false) {
+    if ((this.initialized && !rescan) || !this._directory) {
+      return;
+    }
+    this.initialized = true;
+
     // Use a snapshot of the directory contents to avoid possible issues with
     // iterating over a directory while removing files from it (the YAFFS2
     // embedded filesystem has this issue, see bug 772238).
@@ -8200,7 +8227,9 @@ class DirectoryInstallLocation {
   /**
    * Gets an array of nsIFiles for add-ons installed in this location.
    */
-  getAddonLocations() {
+  getAddonLocations(rescan = false) {
+    this._readAddons(rescan);
+
     let locations = new Map();
     for (let id in this._IDToFileMap) {
       locations.set(id, this._IDToFileMap[id].clone());
@@ -8217,6 +8246,9 @@ class DirectoryInstallLocation {
    * @throws if the ID does not match any of the add-ons installed
    */
   getLocationForID(aId) {
+    if (!(aId in this._IDToFileMap))
+      this._readAddons();
+
     if (aId in this._IDToFileMap)
       return this._IDToFileMap[aId].clone();
     throw new Error("Unknown add-on ID " + aId);