--- a/browser/base/content/test/general/browser_extension_permissions.js
+++ b/browser/base/content/test/general/browser_extension_permissions.js
@@ -1,16 +1,16 @@
 "use strict";
 
 const BASE = getRootDirectory(gTestPath)
   .replace("chrome://mochitests/content/", "https://example.com/");
 
-const PAGE = `${BASE}/file_install_extensions.html`;
-const PERMS_XPI = `${BASE}/browser_webext_permissions.xpi`;
-const NO_PERMS_XPI = `${BASE}/browser_webext_nopermissions.xpi`;
+const INSTALL_PAGE = `${BASE}/file_install_extensions.html`;
+const PERMS_XPI = "browser_webext_permissions.xpi";
+const NO_PERMS_XPI = "browser_webext_nopermissions.xpi";
 const ID = "permissions@test.mozilla.org";
 
 const DEFAULT_EXTENSION_ICON = "chrome://browser/content/extension.svg";
 
 Services.perms.add(makeURI("https://example.com/"), "install",
                    Services.perms.ALLOW_ACTION);
 
 function promisePopupNotificationShown(name) {
@@ -31,67 +31,97 @@ function promisePopupNotificationShown(n
 }
 
 function promiseGetAddonByID(id) {
   return new Promise(resolve => {
     AddonManager.getAddonByID(id, resolve);
   });
 }
 
-function checkNotification(panel, url) {
+function checkNotification(panel, filename) {
   let icon = panel.getAttribute("icon");
 
   let ul = document.getElementById("addon-webext-perm-list");
   let header = document.getElementById("addon-webext-perm-intro");
 
-  if (url == PERMS_XPI) {
+  if (filename == PERMS_XPI) {
     // The icon should come from the extension, don't bother with the precise
     // path, just make sure we've got a jar url pointing to the right path
     // inside the jar.
     ok(icon.startsWith("jar:file://"), "Icon is a jar url");
     ok(icon.endsWith("/icon.png"), "Icon is icon.png inside a jar");
 
     is(header.getAttribute("hidden"), "", "Permission list header is visible");
     is(ul.childElementCount, 4, "Permissions list has 4 entries");
     // Real checking of the contents here is deferred until bug 1316996 lands
-  } else if (url == NO_PERMS_XPI) {
+  } else if (filename == NO_PERMS_XPI) {
     // This extension has no icon, it should have the default
     is(icon, DEFAULT_EXTENSION_ICON, "Icon is the default extension icon");
 
     is(header.getAttribute("hidden"), "true", "Permission list header is hidden");
     is(ul.childElementCount, 0, "Permissions list has 0 entries");
   }
 }
 
+// Navigate the current tab to the given url and return a Promise
+// that resolves when the page is loaded.
+function load(url) {
+  gBrowser.selectedBrowser.loadURI(INSTALL_PAGE);
+  return BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+}
+
 const INSTALL_FUNCTIONS = [
-  function installMozAM(url) {
-    return ContentTask.spawn(gBrowser.selectedBrowser, url, function*(cUrl) {
-      yield content.wrappedJSObject.installMozAM(cUrl);
+  async function installMozAM(filename) {
+    await load(INSTALL_PAGE);
+
+    await ContentTask.spawn(gBrowser.selectedBrowser, `${BASE}/${filename}`, function*(url) {
+      yield content.wrappedJSObject.installMozAM(url);
     });
   },
 
-  function installTrigger(url) {
-    ContentTask.spawn(gBrowser.selectedBrowser, url, function*(cUrl) {
-      content.wrappedJSObject.installTrigger(cUrl);
+  async function installTrigger(filename) {
+    await load(INSTALL_PAGE);
+
+    ContentTask.spawn(gBrowser.selectedBrowser, `${BASE}/${filename}`, function*(url) {
+      content.wrappedJSObject.installTrigger(url);
     });
-    return Promise.resolve();
+  },
+
+  async function installFile(filename) {
+    const ChromeRegistry = Cc["@mozilla.org/chrome/chrome-registry;1"]
+                                       .getService(Ci.nsIChromeRegistry);
+    let chromeUrl = Services.io.newURI(gTestPath);
+    let fileUrl = ChromeRegistry.convertChromeURL(chromeUrl);
+    let file = fileUrl.QueryInterface(Ci.nsIFileURL).file;
+    file.leafName = filename;
+
+    let MockFilePicker = SpecialPowers.MockFilePicker;
+    MockFilePicker.init(window);
+    MockFilePicker.returnFiles = [file];
+
+    await BrowserOpenAddonsMgr("addons://list/extension");
+    let contentWin = gBrowser.selectedTab.linkedBrowser.contentWindow;
+
+    // Do the install...
+    contentWin.gViewController.doCommand("cmd_installFromFile");
+    MockFilePicker.cleanup();
   },
 ];
 
 add_task(function* () {
   yield SpecialPowers.pushPrefEnv({set: [
     ["extensions.webapi.testing", true],
     ["extensions.install.requireBuiltInCerts", false],
 
     // XXX remove this when prompts are enabled by default
     ["extensions.webextPermissionPrompts", true],
   ]});
 
-  function* runOnce(installFn, url, cancel) {
-    let tab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, PAGE);
+  function* runOnce(installFn, filename, cancel) {
+    let tab = yield BrowserTestUtils.openNewForegroundTab(gBrowser);
 
     let installPromise = new Promise(resolve => {
       let listener = {
         onDownloadCancelled() {
           AddonManager.removeInstallListener(listener);
           resolve(false);
         },
 
@@ -113,20 +143,20 @@ add_task(function* () {
         onInstallFailed() {
           AddonManager.removeInstallListener(listener);
           resolve(false);
         },
       };
       AddonManager.addInstallListener(listener);
     });
 
-    let installMethodPromise = installFn(url);
+    let installMethodPromise = installFn(filename);
 
     let panel = yield promisePopupNotificationShown("addon-webext-permissions");
-    checkNotification(panel, url);
+    checkNotification(panel, filename);
 
     if (cancel) {
       panel.secondaryButton.click();
       try {
         yield installMethodPromise;
       } catch (err) {}
     } else {
       // Look for post-install notification
@@ -135,17 +165,16 @@ add_task(function* () {
 
       // Press OK on the post-install notification
       panel = yield postInstallPromise;
       panel.button.click();
 
       yield installMethodPromise;
     }
 
-
     let result = yield installPromise;
     let addon = yield promiseGetAddonByID(ID);
     if (cancel) {
       ok(!result, "Installation was cancelled");
       is(addon, null, "Extension is not installed");
     } else {
       ok(result, "Installation completed");
       isnot(addon, null, "Extension is installed");