--- a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
@@ -1281,28 +1281,29 @@ var AddonTestUtils = {
    * Override chrome URL for specifying allowed built-in add-ons.
    *
    * @param {object} data - An object specifying which add-on IDs are permitted
    *                        to load, for instance: { "system": ["id1", "..."] }
    */
   async overrideBuiltIns(data) {
     // We need to set this in order load the URL preloader service, which
     // is only possible when running in automation.
+    let prevPrefVal = Services.prefs.getBoolPref(PREF_DISABLE_SECURITY, false);
     Services.prefs.setBoolPref(PREF_DISABLE_SECURITY, true);
     aomStartup.initializeURLPreloader();
 
     let file = FileUtils.getFile("TmpD", "override.txt");
     let manifest = Services.io.newFileURI(file);
     await OS.File.writeAtomic(file.path,
       new TextEncoder().encode(JSON.stringify(data)));
     this.overrideEntry = aomStartup.registerChrome(manifest, [
       ["override", "chrome://browser/content/built_in_addons.json",
        Services.io.newFileURI(file).spec],
     ]);
-    Services.prefs.setBoolPref(PREF_DISABLE_SECURITY, false);
+    Services.prefs.setBoolPref(PREF_DISABLE_SECURITY, prevPrefVal);
   }
 };
 
 for (let [key, val] of Object.entries(AddonTestUtils)) {
   if (typeof val == "function")
     AddonTestUtils[key] = val.bind(AddonTestUtils);
 }