--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -2218,17 +2218,17 @@ var AddonManagerInternal = {
                                 Cr.NS_ERROR_INVALID_ARG);
 
   if (!aCallback || typeof aCallback != "function")
     throw Components.Exception("aCallback must be a function",
                                Cr.NS_ERROR_INVALID_ARG);
 
    this.getAddonByInstanceID(aInstanceID).then(wrapper => {
      if (!wrapper) {
-       throw Error(`No addon matching instanceID: ${aInstanceID}`);
+       throw Error(`No addon matching instanceID: ${String(aInstanceID)}`);
      }
      let addonId = wrapper.id;
      logger.debug(`Registering upgrade listener for ${addonId}`);
      this.upgradeListeners.set(addonId, aCallback);
    });
   },
 
   /**

--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -1420,17 +1420,17 @@ class XPIStateLocation extends Map {
    *
    * @param {string} addonId
    *        The ID of the add-on represented by the given file.
    * @param {nsIFile} file
    *        The local file or directory containing the add-on.
    * @returns {XPIState}
    */
   addFile(addonId, file) {
-    let xpiState = this._addState(addonId, {enabled: true, file: file.clone()});
+    let xpiState = this._addState(addonId, {enabled: false, file: file.clone()});
     xpiState.getModTime(xpiState.file, addonId);
     return xpiState;
   }
 
   /**
    * Migrates saved state data for the given add-on from the values
    * stored in xpiState and bootstrappedAddons preferences, and adds it to
    * the DB.
@@ -1570,17 +1570,17 @@ this.XPIStates = {
     let changed = false;
     let oldLocations = new Set(Object.keys(oldState));
 
     for (let location of XPIProvider.installLocations) {
       oldLocations.delete(location.name);
 
       // The results of scanning this location.
       let loc = this.getLocation(location.name, location.path || null,
-                                 oldState[location.name]);
+                                 oldState[location.name] || undefined);
       changed = changed || loc.changed;
 
       // Don't bother checking scopes where we don't accept side-loads.
       if (ignoreSideloads && !(location.scope & gStartupScanScopes)) {
         continue;
       }
 
       if (location.name == KEY_APP_TEMPORARY) {

--- a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
@@ -673,26 +673,31 @@ function check_startup_changes(aType, aI
  *          The directory to add the install.rdf to
  * @param   aId
  *          An optional string to override the default installation aId
  * @param   aExtraFile
  *          An optional dummy file to create in the directory
  * @return  An nsIFile for the directory in which the add-on is installed.
  */
 function writeInstallRDFToDir(aData, aDir, aId = aData.id, aExtraFile = null) {
+  return awaitPromise(promiseWriteInstallRDFToDir(aData, aDir, aId, aExtraFile));
+}
+async function promiseWriteInstallRDFToDir(aData, aDir, aId = aData.id, aExtraFile = null) {
   let files = {
     "install.rdf": AddonTestUtils.createInstallRDF(aData),
   };
-  if (aExtraFile)
+  if (typeof aExtraFile === "object")
+    Object.assign(files, aExtraFile);
+  else
     files[aExtraFile] = "";
 
   let dir = aDir.clone();
   dir.append(aId);
 
-  awaitPromise(AddonTestUtils.promiseWriteFilesToDir(dir.path, files));
+  await AddonTestUtils.promiseWriteFilesToDir(dir.path, files);
   return dir;
 }
 
 /**
  * Writes an install.rdf manifest into a packed extension using the properties passed
  * in a JS object. The objects should contain a property for each property to
  * appear in the RDF. The object may contain an array of objects with id,
  * minVersion and maxVersion in the targetApplications property to give target
@@ -707,29 +712,35 @@ function writeInstallRDFToDir(aData, aDi
  * @param   aExtraFile
  *          An optional dummy file to create in the extension
  * @return  A file pointing to where the extension was installed
  */
 function writeInstallRDFToXPI(aData, aDir, aId = aData.id, aExtraFile = null) {
   let files = {
     "install.rdf": AddonTestUtils.createInstallRDF(aData),
   };
+  if (typeof aExtraFile === "object")
+    Object.assign(files, aExtraFile);
+  else
   if (aExtraFile)
     files[aExtraFile] = "";
 
   if (!aDir.exists())
     aDir.create(AM_Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
 
   var file = aDir.clone();
   file.append(`${aId}.xpi`);
 
   AddonTestUtils.writeFilesToZip(file.path, files);
 
   return file;
 }
+async function promiseWriteInstallRDFToXPI(aData, aDir, aId = aData.id, aExtraFile = null) {
+  return writeInstallRDFToXPI(aData, aDir, aId, aExtraFile);
+}
 
 /**
  * Writes an install.rdf manifest into an extension using the properties passed
  * in a JS object. The objects should contain a property for each property to
  * appear in the RDF. The object may contain an array of objects with id,
  * minVersion and maxVersion in the targetApplications property to give target
  * application compatibility.
  *

new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_invalid_install_rdf.js
@@ -0,0 +1,82 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+// Test that side-loaded extensions with invalid install.rdf files are
+// not initialized at startup.
+
+const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
+
+const APP_ID = "xpcshell@tests.mozilla.org";
+
+Services.prefs.setIntPref("extensions.enabledScopes", AddonManager.SCOPE_USER);
+
+createAppInfo(APP_ID, "XPCShell", "1", "1.9.2");
+
+const userAppDir = AddonTestUtils.profileDir.clone();
+userAppDir.append("app-extensions");
+userAppDir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+AddonTestUtils.registerDirectory("XREUSysExt", userAppDir);
+
+const userExtensions = userAppDir.clone();
+userExtensions.append(APP_ID);
+userExtensions.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+
+add_task(async function() {
+  await promiseWriteInstallRDFToXPI({
+    id: "langpack-foo@addons.mozilla.org",
+    version: "1.0",
+    type: 8,
+    targetApplications: [{
+      id: "xpcshell@tests.mozilla.org",
+      minVersion: "1",
+      maxVersion: "1"
+    }],
+    name: "Invalid install.rdf extension",
+  }, userExtensions, undefined, {
+    "chrome.manifest": `
+      resource foo-langpack ./
+    `,
+  });
+
+  await promiseWriteInstallRDFToXPI({
+    id: "foo@addons.mozilla.org",
+    version: "1.0",
+    targetApplications: [{
+      id: "xpcshell@tests.mozilla.org",
+      minVersion: "1",
+      maxVersion: "1"
+    }],
+    name: "Invalid install.rdf extension",
+  }, userExtensions, undefined, {
+    "chrome.manifest": `
+      resource foo ./
+    `,
+  });
+
+  const resProto = Services.io.getProtocolHandler("resource");
+  resProto.QueryInterface(Ci.nsIResProtocolHandler);
+
+  equal(resProto.hasSubstitution("foo-langpack"), false,
+        "Should not foo-langpack resource before AOM startup");
+  equal(resProto.hasSubstitution("foo"), false,
+        "Should not foo resource before AOM startup");
+
+  await promiseStartupManager();
+
+  equal(resProto.hasSubstitution("foo-langpack"), false,
+        "Should not have registered chrome manifest for invalid extension");
+  equal(resProto.hasSubstitution("foo"), true,
+        "Should have registered chrome manifest for valid extension");
+
+  await promiseRestartManager();
+
+  equal(resProto.hasSubstitution("foo-langpack"), false,
+        "Should still not have registered chrome manifest for invalid extension after restart");
+  equal(resProto.hasSubstitution("foo"), true,
+        "Should still have registered chrome manifest for valid extension after restart");
+
+  await promiseShutdownManager();
+
+  userAppDir.remove(true);
+});

--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
@@ -15,16 +15,17 @@ 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_hotfix_cert.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]
 [test_ProductAddonChecker.js]