--- a/toolkit/mozapps/extensions/test/xpcshell/test_temporary.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_temporary.js
@@ -818,31 +818,38 @@ add_task(async function() {
   equal(addon.temporarilyInstalled, false);
 
   await promiseRestartManager();
 });
 
 // Check that WebExtensions experiments can only be installed temporarily
 // in builds that allow legacy extensions.
 add_task(async function() {
+  AddonTestUtils.usePrivilegedSignatures = false;
+
   const API_ID = "apiexperiment@tests.mozilla.org";
   let xpi = createTempXPIFile({
     id: API_ID,
     name: "WebExtension Experiment",
     version: "1.0",
     type: 256,
     targetApplications: [{
       id: "xpcshell@tests.mozilla.org",
       minVersion: "1",
       maxVersion: "1"
     }],
   });
 
-  await AddonManager.installTemporaryAddon(xpi);
-  let addon = await promiseAddonByID(API_ID);
+  let addon = null;
+  try {
+    await AddonManager.installTemporaryAddon(xpi);
+    addon = await promiseAddonByID(API_ID);
+  } catch (err) {
+    // fall through, level addon null
+  }
 
   if (AppConstants.MOZ_ALLOW_LEGACY_EXTENSIONS) {
     notEqual(addon, null, "Temporary install of WebExtension experiment succeeded");
     addon.uninstall();
   } else {
     equal(addon, null, "Temporary install of WebExtension experiment was not allowed");
   }
 });