--- a/toolkit/mozapps/extensions/internal/AddonRepository.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonRepository.jsm
@@ -116,37 +116,47 @@ function convertHTMLToPlainText(html) {
   converter.convert("text/html", input, input.data.length, "text/unicode",
                     output, {});
 
   if (output.value instanceof Ci.nsISupportsString)
     return output.value.data.replace(/\r\n/g, "\n");
   return html;
 }
 
-function getAddonsToCache(aIds) {
+async function getAddonsToCache(aIds) {
   let types = Preferences.get(PREF_GETADDONS_CACHE_TYPES) || DEFAULT_CACHE_TYPES;
 
   types = types.split(",");
 
-  return AddonManager.getAddonsByIDs(aIds).then(addons => {
-    let enabledIds = [];
-    for (let [i, addon] of addons.entries()) {
-      var preference = PREF_GETADDONS_CACHE_ID_ENABLED.replace("%ID%", aIds[i]);
-      // If the preference doesn't exist caching is enabled by default
-      if (!Preferences.get(preference, true))
-        continue;
+  let addons = await AddonManager.getAddonsByIDs(aIds)
+  let enabledIds = [];
 
-      // The add-ons manager may not know about this ID yet if it is a pending
-      // install. In that case we'll just cache it regardless
-      if (!addon || types.includes(addon.type))
-        enabledIds.push(aIds[i]);
+  for (let [i, addon] of addons.entries()) {
+    var preference = PREF_GETADDONS_CACHE_ID_ENABLED.replace("%ID%", aIds[i]);
+    // If the preference doesn't exist caching is enabled by default
+    if (!Preferences.get(preference, true))
+      continue;
+
+    // The add-ons manager may not know about this ID yet if it is a pending
+    // install. In that case we'll just cache it regardless
+
+    // Don't cache add-ons of the wrong types
+    if (addon && !types.includes(addon.type)) {
+      continue;
     }
 
-    return enabledIds;
-  });
+    // Don't cache system add-ons
+    if (addon && addon.isSystem) {
+      continue;
+    }
+
+    enabledIds.push(aIds[i]);
+  }
+
+  return enabledIds;
 }
 
 function AddonSearchResult(aId) {
   this.id = aId;
   this.icons = {};
   this._unsupportedProperties = {};
 }
 

new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
@@ -0,0 +1,52 @@
+// Tests that AddonRepository doesn't download results for system add-ons
+
+const PREF_GETADDONS_CACHE_ENABLED = "extensions.getAddons.cache.enabled";
+
+BootstrapMonitor.init();
+
+Components.utils.import("resource://testing-common/httpd.js");
+var gServer = new HttpServer();
+gServer.start(-1);
+
+// Build the test set
+var distroDir = FileUtils.getDir("ProfD", ["sysfeatures"], true);
+do_get_file("data/system_addons/system1_1.xpi").copyTo(distroDir, "system1@tests.mozilla.org.xpi");
+do_get_file("data/system_addons/system2_1.xpi").copyTo(distroDir, "system2@tests.mozilla.org.xpi");
+do_get_file("data/system_addons/system3_1.xpi").copyTo(distroDir, "system3@tests.mozilla.org.xpi");
+registerDirectory("XREAppFeat", distroDir);
+
+createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "0");
+
+function getCachedAddon(id) {
+  return new Promise(resolve => AddonRepository.getCachedAddonByID(id, resolve));
+}
+
+// Test with a missing features directory
+add_task(function* test_app_addons() {
+  Services.prefs.setBoolPref(PREF_GETADDONS_CACHE_ENABLED, true);
+  Services.prefs.setCharPref(PREF_GETADDONS_BYIDS, `http://localhost:${gServer.identity.primaryPort}/get?%IDS%`);
+
+  gServer.registerPathHandler("/get", (request, response) => {
+    do_throw("Unexpected request to server.");
+  });
+
+  startupManager();
+
+  yield new Promise((resolve) => {
+    AddonRepository.cacheAddons(["system1@tests.mozilla.org",
+                                 "system2@tests.mozilla.org",
+                                 "system3@tests.mozilla.org"], resolve);
+  });
+
+  let cached = yield getCachedAddon("system1@tests.mozilla.org");
+  do_check_eq(cached, null);
+
+  cached = yield getCachedAddon("system2@tests.mozilla.org");
+  do_check_eq(cached, null);
+
+  cached = yield getCachedAddon("system3@tests.mozilla.org");
+  do_check_eq(cached, null);
+
+  yield promiseShutdownManager();
+  yield new Promise(resolve => gServer.stop(resolve));
+});

--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
@@ -27,16 +27,17 @@ 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]
 [test_shutdown.js]
 [test_system_update.js]
 [test_system_update_fail.js]
+[test_system_repository.js]
 [test_system_reset.js]
 [test_XPIcancel.js]
 [test_XPIStates.js]
 [test_temporary.js]
 tags = webextensions
 [test_install_from_sources.js]
 [test_proxies.js]
 [test_proxy.js]